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Qhc TRural QcxUBook Series 

Edited by L. H. BAILEY 



PLANT PHYSIOLOGY 

WITH SPECIAL REFERENCE TO 

PLANT PRODUCTION 



2Tiir Eural Ccit-Book Series 

Lyon and Fippin, The Principles of Soil 
Management. 

Warren. Elements of Agriculture. 

J. F. Duggar, Southern Field Crops. 

B. M. Duggar, Plant Physiology. 

Others in preparation. 



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COPTBIOHT, 1911, 

V.\ Till. MAC Mil. LAN COMPANY. 



Set up and electxotyped. Published May i v ii 



Xortooot) JjJrrsg 

i. B. Cubing <'■•. Berwick A Bmltb <<>. 

Norwood, Uiu., D.B.i 



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PREFACE 

In the preparation of this text and reference book, the 
writer has attempted to consider both the student and 
the general reader, interested alike in the fundamental 
requirements of plants and in plant production. Through- 
out biological study at the present time increased emphasis 
is placed on the activities and responses of organisms. 
It is instruction in this type of biological phenomena that 
is rapidly becoming a part of the cultural side of education, 
and the practical value of such knowledge is every day 
being demonstrated, — notably in agriculture and medicine. 
Plant physiology finds its practical application in plant 
production, to which it stands in much the same relation 
as does industrial chemistry to general manufacturing. 

It is somewhat strange, therefore, to find that as a sepa- 
rate course plant physiology is not yet offered in some of 
the colleges whose purpose is primarily to train persons 
for practical or rural pursuits. Such students require some 
fundamental work, and few will become specialists. For 
this general class of students, and for other readers as well, 
there seems to be needed a text (1) that shall exhibit a con- 
siderable range of material, rather than a few topics ex- 
haustively treated; (2) that shall include both qualitative 
and quantitative work; and (3) that shall keep in view, 
as far as possible, the relations of the science to plant 
production, drawing the illustrations, wherever convenient, 
v 



vi /'/' 

from plants which are familiar and directly useful. By 
maintaining Borne direcl contacl with practical problems, 
interest is aroused for further desirable fundamental prepa- 
ration. "The idea that useful knowledge cannol be cultural 
must be dismissed. . . . Every possible application must 
be made of each abstract principle." (Eliot, " The Conflict 
between Individualism and Collectivism in a Democracy, 

In the field of pure physiology, there are recent texts 
and guides embodying much of what is considered best 
in tlif modern contenl and attitude of the Bcience. An 

ration of the methods of quantitative stu.lv is there 
indicated, and stress is laid on the materials and energy 
involved in plant activity. Such books will be consulted 
with much profit 

In selecting from the great amount of available material 

that which lias seemed to he most suitable tor the present 
purpose, consideration has been given the fact that in many 
colleges genera] courses are offered, not only in such dis- 
tinctively plant line, a- agronomy, horticulture, and breed- 
ing, but likewise in fields overlapping physiology, or partially 
included in this subject, such as soils, bacteriology, pathol- 
. and genetics. The subject-matter included is intended 
to be Bufficienl tor a course <>f one-hall year involving two 

re.-itations and two laboratory periods; but it may he 
maxl - of a shorter course by suitable .selection of 

material, or of a Longer course by an extension of the col- 
lateral work. 

In the preparation of this text I have used freely any 
available I information. 'I'he subject-matter has 

been presented at one time or another in class work. I am 
indebted to Mr. Lewis Knudson, Instructor in PJant Physi- 



Preface vii 

ology in Cornell University, for many suggestions and for 
the form of certain sections of the laboratory notes. Some 
of the illustrations were furnished by others, or borrowed, 
as credited in the text. Certain of the drawings were pre- 
pared by Miss Anna M. Keichline ; others by Mrs. B. M. 
Duggar, of whose constant assistance with manuscript and 
proof I would express also my appreciation. 

B. M. DUGGAR. 



CONTENTS 

CHAPTER I 
Introduction 

PAGES 

Permanent high production — The relation of physiology to pro- 
duction — Physiology and ecology — Physiological processes 

— Environment — Crop ecology — Literature of Plant Physi- 
ology — Physiology and other sciences — References . 1-14 

CHAPTER II 

The Plant Cell 

The cell a physiological unit — Early use of the term " cell'' — Meri- 
stem or embryonic cells — Cytoplasm — The nucleus — Plas- 
tids — The cell- wall — Cell-sap — Cell-forms — Parenchyma 
Sclerenchyma — Trachea; or vessels — Sieve tubes — Proto- 
plasmic movement — Protoplasmic irritability and response 

— Laboratory work — References 15—34 

CHAPTER III 

The Water-content of Plants and the General Relations 
OF Root Systems 

Hydrostatic rigidity — The water-content of plants — Variation 
in water-content of organs — The water-absorbing system — 
The root habit of crops — The production of root-hairs — 
Root-hairs and the water-content of the soil — The root-cap 

— Structure of the root-tip — Soil particles — Soil texture 



x Contents 

PAGES 

and water-holding capacity — Exceptional plants — Unavail- 
able water — Leaves poorly fitted for water absorption — 
Laboratory work — References 35-63 

CHAPTER IV 
Conditions and Principles of Absorption 

Imbibition phenomena — Osmosis and diffusion — The demon- 
stration of osmotic pressure — An explanation of osmotic 
pressure — Plasmolysis and wilting — Variation in turgor — 
Substances active in producing turgor — Osmosis and the 
absorption of nutrient salts — Protoplasmic permeability — 
The r61e of diffusion and osmotic pressure — Sap or root 
pressure — Laboratory work — References . . . 04-83 

CHAPTER V 
Transpiration and Water Movement 

Observations upon transpiration — Amount of transpiration — 
The mechanism permitting transpiration — Distribution of 
stomata — The control of water-loss by stomatal movement 
— Modifications tending to check excessive transpiration — 
Conditions affecting transpiration — Effects of excessive 
evaporation — Guttation — Transpiration and evaporation — 
Transpiration and growth — Water transport — Fibrovas- 
cular bundles — Leaf venation — Rate of transport — Labora- 
tory work — References 84-115 

CHAPTER VI 
The Water Requirements of Crops and of Vegetation 

Relative requirements of a few crops — Precipitation and crop 
growth — Irrigation — Potted plants and water-supply — 
Ecological classification based upon the water relation — 
Semi-xerophytism and hard-wheat production — Subsidiary 
work — References 116-136 



Contents xi 

CHAPTER VII 
Mineral Nutrients 

PAGES 

The ash content of plants — Composition of the ash — Effects of 
conditions upon ash content — Ash content at different ages 

— Translocation of mineral substances — Water cultures — 
Nutrient solutions and water cultures — Strength of the 
nutrient solution — The forms of the nutrient compounds — 
Plant nutrients in rock — Soil fertility — Nutrients removed 
by farm crops — Nutrients removed by fruit crops — Amount 
of nutrients in soils — Availability of the nutrients — The 
solvent action of roots — C0 2 excretion and the availability 
of phosphorus — Another view of soil fertility — The paraf- 
fined wire basket in nutrition studies — Laboratory work — 
References 136-168 

CHAPTER VIII 
Special Functions and Relations of Mineral Nutrients 

The roles of mineral nutrients : The nature of the special roles 

— The role of phosphorus — The role of potassium — The 
role of magnesium — The role of calcium — Iron — Sodium 

— Chlorine — Sulfur — Silicon — References . . . 169-183 
Balanced solutions : The injurious action of certain basic nutri- 
ents — The relation of calcium to magnesium — Other 
nutrient bases and antitoxic action — Laboratory work — 
References 181-194 

CHAPTER IX 

The Intake of Carbon and the Making of Organic Food 

The amount of carbon in the plant — Carbon dioxid the source 
of carbon in green plants — Chlorophyllous plants — Re- 
specting the distribution of chlorophyll — The nature and 
properties of chlorophyll — The factors essential in photo- 



xii Contents 



PAGES 



synthesis — The course of photosynthesis — The demonstra- 
tion of photosynthesis — The formation of sugar and starch 
— The diffusion process — The amount of carbon dioxid — 
Light the source of energy — Efficiency of the food-making 
apparatus — Light, intensity and quality — Temperature — 
Organic matter, rate of production — Laboratory work — 
References 195-225 



CHAPTER X 
The Relation to Nitrogen 

Combined nitrogen — The nitrogen content of plants — Synthesis 
of nitrogenous bodies — Soil nitrogen — Nitrites — Nitrates 
— Compounds of ammonium — The sources of soil nitrates 
and ammonia — Ammonification — Nitrification — Nitrifying 
organisms — Conditions favoring nitrification — Denitrifica- 
tion — Nitrogen fixation — Organisms which fix free nitro- 
gen Bacteria of leguminous tubercles — Certain saprophytic 

soil bacteria— Fungi — Mycorhizal fungi — General sources 
of supply of nitrogen — Electric fixation of nitrogen — 
Laboratory work — References 226-249 



CHAPTER XI 
Products of Metabolism ; Digestion and Translocation 

Metabolism — Temporary foods, storage products, and perma- 
nent structures — Annuals, biennials, and perennials — Car- 
bohydrates — Sugars — Starches — Cellulose — Fats and oils 

— Proteins — Classes of proteins — Amides — Organic acids 

— Tannins — Resins and turpentine — Digestion — Diges- 
tion in different organisms — Enzymes and enzyme action — 
Carbohydrate enzymes and their products — Protein enzymes 

— Conduction of digested foods — Ringing — Laboratory 
work — References 250-279 



Contents xiii 

CHAPTER XII 

Respiration, Aeration, and Fermentation- 
pages 
The term "respiration '' — An obvious result of respiration — 

The demonstration of respiration — Respiratory phenomena 
in aerobic respiration — Oxygen promotes catabolic processes 

— The ratio of 2 absorption to C0 2 production — Respira- 
tory activity — Respiration of wounded plants — Heat re- 
lease — The mechanism for gas exchange — Anaerobic 
respiration — Fermentation — Lactic fermentation — Alco- 
holic fermentation — Acetic fermentation — Laboratory work 

— References 280-304 

CHAPTER XIII 

Growth 

The factors — Evidences of growth — Growth of the embryo — 
Polarity — Elongation of roots — The stem apex — The for- 
mation and exfoliation of leaves — The resting bud — Types 
of stem elongation — Fruit buds and age of shoot — Persist- 
ence of the rest period in temperate regions — Differentia- 
tion of stem tissues — Secondary thickening — Growth of 
the cell — Cell division — Nuclear division — Cell division 
and respiration — The relation of pruning to growth — Bud- 
ding and grafting — Scion propagation — Relation of stock 
to scion — Forcing — Etherization — The effect of etheriza- 
tion — Forcing by immersion in warm water — Transplanting 
after wilting — Growth movements — Laboratory work — 
References 305-34G 

CHAPTER XIV 

Reproduction 

The seed habit and vegetative production — The flower : essential 
structures — Pistil and stamen — Pollination and pollen-tube 
penetration — Fertilization — Universality of fertilization — 
Cross-fertilization and self-fertilization — Cross-fertilization 



xiv Contents 



PAGES 



apparently the rule — Darwin's conclusions — The need of 
further work — Experiments with self -sterility in pear — 
Self-sterility in other orchard trees — Parthenogenesis — 
Xenia in corn — Indications of xenia — False xenia — Other 
secondary effects of pollination — Parthenocarpic develop- 
ment — Parthenocarpic formation in pomaceous fruits — 
Seedlessness in the orange, grape, and banana— Nonsexual 
reproduction — Thickened roots and tubers — Cuttings — 
Precautions with cuttings — Vegetative reproduction and 
running out — Relation of vegetation to fruiting — Labora- 
tory work — References 347-380 

CHAPTER XV 

The Seed in Plant Production 

Habitat conditions of the parent plant — Localization of seed 
production — Maturity — Conditions of harvesting and cur- 
ing _ Duration of vitality — Environmental conditions — 
Buried seed — Delayed germination — Effect of weight and 
size of seed upon vigor — Experiments with wheat — Ex- 
periments with cotton — Experiments with tobacco — Lab- 
oratory or supplementary work — References . . 381-399 

CHAPTER XVI 

The Temperature Relation 

Climatic extremes and introduced plants — Temperature and 
production — Cardinal temperatures — Inhibition at high 
temperatures — Heat units— Heat units and germination — 
The date-palm — Control of temperature — The temperature 
of the plant — Adjustment of structure — Irritable response 
— Freezing — Buds — Laboratory work — References . 400-414 

CHAPTER XVII 

The Light Relation 

The adjustment of plant members — Light perception — Diverse 
requirements — Light intensity — Injurious effects — Artifi- 



Contents xv 

PAGES 

cial light — Monochromatic light — Half -shade in plant 
propagation — Crops responding to half-shade — Morpho- 
geny effects — Half-shade and quality — The effect of shad- 
ing upon other environmental factors — Laboratory work — 
References 415-435 

CHAPTER XVIII 
Relation to Deleterious Chemical Agents 
General relations to poisons — Comparative resistance — Toxic 
action and the substratum — Method of action — Inorganic 
and organic acids — Alkalies — Salts of the heavy metals — 
Formalin — Organic bodies — Root excretions — Unproduc- 
tiveness — Relative toxicity of some organic compounds — 
Illuminating gas — Stimulation by means of weak toxic 
agents — Protection of crops by insecticides and fungicides 

— Destruction of weeds by poisons — Deleterious substances 
employed — Practicability of the chemical method — Labora- 
tory work — References 436-462 

CHAPTER XIX 

Variation and Heredity 

Variation: Individuals and species — Fluctuating variations — 
Darwin's theory of natural selection — Rate of increase — 
Fluctuating variation and the origin of varieties — Pure lines 

— Mutations — Mutation and crop improvement . . 463-476 
Heredity : Nonsexual reproduction and heredity — Sexual repro- 
duction and heredity — The early studies — Types of inherit- 
ance — Recent studies — Mendel's experiments — Purity of 

the gametes — Results of segregation — Tomato characters 

— Chromosome relations — Selection — Laboratory work — 
References 476-493 

CHAPTER XX 
Growth Movements 
Stimulus and response — Tropic curvatures — Geotropism — Thig- 
motropism — Chemotropism — Nutation — Nastic curvatures 

— Nyctitropism — Laboratory work — References. . 494-507 



PLAKT PHYSIOLOGY 



CHAPTER I 
INTRODUCTION 

The relation of plant physiology to crop production and 
vegetation requires no explanation except where physiology 
and plant production are alike incompletely comprehended. 
No one may thoroughly understand a modern agricultural 
problem who has not learned the full significance and 
scientific relations of the two eminently practical terms 
" production" and " conservation." The basal field of 
agriculture is plant production, for upon this animal pro- 
duction is dependent ; and throughout all agriculture con- 
servation is necessarily the key to continuous development 
and success. 

Conservation in the broadest sense implies neither 
waste of the product grown nor waste of the forces and 
conditions which make high production possible. These 
forces are the environment under which the crop is grown 
and the inherent hereditary possibilities within the seed 
or seed-material. 

1. Permanent high production. — Plants form the nat- 
ural covering of the surface of the earth, and if there is 
at present no such covering, where the rock is sufficiently 
b 1 



2 Plant Physiology 

decomposed to be termed soil, it indicates that something 
is radically wrong with the soil or climate of that region 
from the standpoint of the permanent occupation of it by 
man or animal as well as by plants. 

Of all object lessons in permanent occupation by plants, 




Fig. 1. In the Rainier National Forest, Washington. [Photograph from 
the Forest Service.] 

that of the aged forest stands out supreme. Here a vigor- 
ous growth may have endured for centuries, and except 
for such accidents as those of floods and fires, or of fla- 
grant devastation by man, it might continue for centuries 
more. So far as may be seen, or measured by the short 
space of agricultural record, at least, there is with the 
greater growth of the forest an ever increasing fertility of 
the land. 



Introduction 3 

From this lesson of production and conservation one 
turns to another in discouraging contrast, — and that 
other is this : much of this fertile forest land has been 
cleared, and year by year fields which were once highly 
productive are left untilled, or abandoned as of no longer 
interest agriculturally. If lessened production is the cause 
of abandonment or discouragement, the system which 
leads to waste of this nature should have a speedy end. 

All of the results of science and practice are needed to 
assist in working a change in the conditions. Each science 
may contribute something. 

2. The relation of physiology to production. — Plant 
physiology is an intimate part of scientific plant production. 
It concerns itself with plant response and plant behavior 
under all conditions ; that is, with all relations and processes 
readily evident or obscure, simple or complex, which have 
to do with the maintenance, growth, and reproduction of 
plants. It is then concerned with vegetation or crops, 
with the relation of the plant as a whole, and with all 
special responses or functions of any organ or cell. From 
the standpoint of physiology one should be able to get 
facts alike applicable in understanding or interpreting 
the behavior or yield of plants of all description. The 
principles of growth are learned by the same methods, 
whether the plants are those constituting the vegetation of 
the mill-pond or of the vast fields of cultivated grain; of 
the greenhouse or of the weedy growth of the neglected 
lot ; of the sparse vegetation of the poor prairie or of the 
primeval forest. Throughout all, the principles involved 
are ultimately those of analyzing the complex stimuli and 
the resulting growth, or maintenance, and reproduction. 



4 Plant Physiology 

Through physiological study it is possible to understand 
better that which pertains to production, since increased 
knowledge of plant response makes it more nearly possible 
to modify opportunely and to improve upon current prac- 
tices of production, and to develop progressive varieties 
and strains. Obviously, production involves a variety of 
nonphysiological conditions, but it also involves physio- 
logical conditions, and little progress may be anticipated 
without an intimate knowledge of the relation of the 
growth of the plant or crop to the conditions under which 
it is grown. 

3. Physiology and ecology. — At the outset, moreover, 
it is necessary to recognize two possible lines of study and 
observation. The one is primarily concerned with the 
isolated and controlled plant and the functions or responses 
of its diverse organs and structures. This is generally 
considered pure physiology. The other line of study deals 
with plants or the crop in the field, or stated technically, in 
a natural or seminatural habitat. This is field physiology 
or ecology. There is, of course, no sharp line between 
the two subdivisions indicated, and both are important in 
production. It is necessary to know the plant, and it is 
equally essential to know the environment, for that is the 
sum of the conditions to which the plant responds. 

4. Physiological processes. — The engineer who does 
not understand his machine cannot expect to get effective 
work out of it. He should know its intimate structure, 
what work it can perform under all conditions, and how it 
may be controlled. In the same way the plant producer 
who knows the structure of the plant and its behavior is 
provided with the means of interpreting the effects of con- 



Introduction 5 

ditions upon the organism. The plant is a delicate physi- 
cal, chemical, and living mechanism; and through the 
work which is performed, often expressed in growth, or 
change of some sort, it is responsive to practically all 
manner of stimuli. 

Under whatever conditions it may be able to grow and 
reproduce itself there are many phenomena, or processes, 
which are recognized as fundamentally physiological, al- 
though the information respecting these may often be es- 
sentially physical or chemical. All modern physiologists 
are necessarily pursuing physico-chemical methods in in- 
terpreting all the activities of plants. Among these pro- 
cesses may be mentioned the absorption, movement, and 
incorporation of water and of gases ; the absorption and 
disposition of nutrient salts; the manufacture of organic 
food material ; the accumulation, digestion, and assimila- 
tion of foods or food materials ; respiration ; growth and 
variability; reproduction and heredity; and the special 
growth or other changes as responses to environmental 
factors. Much of the material presented in this book is 
for the purpose of demonstrating simply some of the essen- 
tial principles involved. Qualitative measurements are 
frequently sufficient. 

5. Environment. — The environment of any plant or 
crop is a complex of factors or conditions as a resultant of 
which there is the response of the plant in vigorous or 
weakly production, and in diverse form or habit of growth. 
Most of the important factors of the environment affecting 
the agricultural plant are perfectly obvious. In consider- 
ing these one almost unconsciously thinks of those factors 
which operate above the soil and those which operate 



6 Plant Physiology 

through the soil. This division of the factors is not wholly 
satisfactory, as will be seen. Agronomy is commonly sub- 
divided nowadays into " soils " and " crops " ; but these 
do not exclude physiology, they make it, in its broadest 
relations, more nearly indispensable. 

Conditions acting above the soil are mainly sunshine, 
heat, precipitation, humidity, evaporation, and air move- 
ment and composition ; while through the soil there is af- 
forded fixity, mineral nutrients, and water, as well as heat 
and air. There are, however, many other factors acting 
above or below the soil which may affect vegetation directly 
or indirectly, including the bacterial flora of the soil, and 
injurious substances in the soil solution; fungous diseases, 
insect pests, and higher animals ; the constant factor grav- 
ity; and many special stimuli. 

This complexity of environmental factors renders an 
interpretation of the effects of any one factor in terms 
of plant behavior peculiar ly difficult. Progress in field 
study, or experimental ecology, is nevertheless being made, 
and much is due to the greater perfection in instrumenta- 
tion as applied to the habitat. 

In nature a species of plant thrives best where the re- 
quirements of the particular form are most completely 
met. In the case of the cultivated plant, however, while 
the natural factors condition production, these alone are, 
of course, insufficient to determine whether or not a par- 
ticular crop should be grown in a particular locality, for 
there are a host of economic considerations which must 
have weight. Market, transportation, labor, machinery, 
and many other factors must be taken into account. 

From the point of view of the natural factors there are 



Introduction 7 

again two lines of inquiry : (1) Is the crop suited to the 
general conditions of the region? (2) Are the conditions 
of environment and the cultural practices afforded the 
crop such as to result in maximum yield? It is not ex- 
pected that any considerable number of facts enabling one 
better to answer the first question shall be presented in 
this book; such facts, at any rate, may be only incidentally 
touched upon. The fundamental facts developed should, 
however, enable one to observe, test, and answer for himself 
more completely any question which would fall in the sec- 
ond group of inquiry. 

6. Crop ecology. — Since ecological data may be in- 
cluded only incidentally, a few words may be said here 
concerning the general relations of plants as distributed 
over the surface of the earth or as cultivated under special 
conditions. In the steppes of northern Africa and portions 
of Australia, in the dry prairies of the western United States 
and southern Russia, or in the equivalent regions of west- 
ern Brazil, the vegetation is similar in physiognomy. Here 
tough and drought-resistant grasses thrive, and for the 
most part these regions are treeless tracts where rains fall 
infrequently or precipitation is poorly distributed, and 
here, too, winds often exert their highest force. The great 
permanent grass lands, such as these areas are, may be 
considered as being ecologically most closely related to true 
desert. 

On the other hand, in tropical or temperate regions of 
abundant, or at least sufficient, rainfall forests of one type 
or another find a natural home. It is clear that upon the 
discovery of North America, the region now included in the 
United States was, practically speaking, a continuous forest 



8 Plant Physiology 

extending from the Atlantic westward to the region of little 
precipitation. 

Agriculture and commerce have already encroached to 
an enormous extent upon the natural domain of both the 
native forest and grazing lands; but in mountainous 
regions and towards the northern limit of vigorous growth 



Fig. 2. Merriam's Life Zones of the United States ; Boreal (1), Transi- 
tion (2), Upper and Middle Austral (3), Lower Austral (4), Gulf strip 
of Lower Austral (5), and Tropical (6). Dotted parts of the Austral zones 
indicate humid divisions. [After Cockerell, in Bailey's Cyclo. Agr.] 

most small crops become less profitable, and the forest will, 
through many generations, at least, form the natural 
boundary line separating agriculture from the Arctic zone. 
In addition, of course, forests will continue to thrive in the 
agricultural region where permitted by man. 

Furthermore, in taking a bird's-eye view of the forests 
of the United States there is noticed a more or less striking 
limitation in range, hence in general adaptability, of many 



Introduction 9 

well-known forest species. Thus the range of white pine 
as a commercial crop is practically limited to a region ex- 
tending westward from the New England States to Min- 
nesota, while the long-leaf yellow pine is restricted to the 
sandy coastal region of the Southern Atlantic and certain 
Gulf States. 

A similar relation of particular crops to one or more 
factors of the environment is strikingly brought out by 
those crops especially that are commonly associated with 
southern climates. Cotton has a relatively long season of 
growth, and it is restricted in the United States to a region 
practically below the thirty-seventh parallel, — a region 
which is, for about seven or eight months of the year, free 
from frost and with a high mean temperature. Citrus 
fruits are accustomed to an almost continuous growing 
season, where frosts are few and severe freezing practically 
unknown. 

A large proportion of the varieties of rice, also restricted 
to warm regions, may not be grown beyond those sections 
in which irrigation is possible. Hard wheats gradually 
lose the quality of " hardness " (high per cent of gluten) 
when grown in moist regions. The potato is grown from 
Canada to Texas and from Scotland to Italy. It is inter- 
esting to note, however, that in the United States, usualfy, 
the yield diminishes toward the South ; and, except under 
special conditions, the crop matures relatively early 
throughout the United States. Under such conditions 
average production is but about 85 bushels per acre. 
With intensive culture 400 bushels is a maximum for some 
of the most productive lands in the eastern United States, 
although 1000 bushels have been reported under peculiar 



10 Plant Physiology 

conditions in the far West. In Scotland, with its more or 
less continued cool climate, affording a long growth and 
slow maturity of the potato, we find an average of nearly 
250 bushels per acre ; while a maximum of 1000 to 
1200 bushels is commonly attained. At the well-known 
seed farm of Lord Rosebery a yield at the rate of over 1700 
bushels per acre was reported for a particular plot during 
the past season. Such facts as these cannot fail to be sug- 
gestive from the ecological standpoint. 

If the more fundamental lines of general physiology 
seem less a part of plant production or of practical agricul- 
ture than the broader relations above referred to, it must be 
that it is so partly because of the name which has been ap- 
plied to this subject, and partly to the fact that the meth- 
ods of instruction necessarily take the student or reader 
to a far greater degree away from the cultivated field. 
To a considerable extent this is necessary, for physiology 
must remain one of the fundamental sciences, and the fun- 
damental attitude should be kept prominent. It has been 
considered, too often, a subject with merely laboratory 
applicability. This erroneous view is vanishing as plant 
producers become more and more interested in the causes 
which produce results and not merely in the results them- 
selves. Both horticultural and agronomic work have in 
recent years extended more and more into the realm of 
pure plant physiology, which should mean that they have 
extended into that of accurate experimental study, with 
the plant response as the central feature. 

7. The literature of plant physiology. — The litera- 
ture of this subject is extensive and scattered, as is that 
of any other science. The student will do well to bear in 



Introduction 11 

mind that while both the brief and the extensive standard 
works are important, the subject is one which, through its 
diverse relationships, encourages breadth of preparation 
and of application ; so that frequently physiological texts 
alone are insufficient. Any standard text is in large part a 
logical arrangement and correlation of the facts of many 
separate papers or monographs; and the detailed data 
respecting any phenomenon should be sought in the spe- 
cial paper. 

The rapid strides which have been made in scientific 
agriculture and horticulture, especially in plant chemistry, 
soils, and intensified production, have developed a great 
array of interesting phenomena. This has given a decided 
impetus to physiological study. Often, unfortunately, the 
agriculturist has been compelled to go forward without due 
knowledge of physiology in the interpretation of his results, 
but this is no excuse for the neglect of the large amount of 
valuable and sound work which has been done. 

Again, it should be further emphasized that many phys- 
iological phenomena are only properly understood when 
they are viewed in the light of physical and chemical the- 
ory, and it is frequently necessary that the student who is 
encouraged to go further shall turn to the sources of in- 
formation in these fundamental sciences. 

8. Physiology and other sciences. — The aim of plant 
physiology is a definite one, like that of other sciences; it 
is ultimately to obtain precise information concerning all 
those factors and forces which are operative within or 
through the living plant. Facts are derived and laws es- 
tablished in exactly the same manner as in other sciences. 
It does not stand apart from physics and chemistry, but 



12 Plant Physiology 

utilizes and advances these or any other sciences which 
may assist in deducing the facts of plant life. Just as 
chemistry may utilize the plant as an indicator of chemical 
reaction or chemical fact, so physiology may use or develop 
chemical facts in analyzing the phenomena of plant life. 

In general, a study of physiology must assume or in- 
clude facts regarding the form and structure of plants; 
that is, morphology and histology. The more elaborate 
the morphology of an organism, as a rule, the more special- 
ized and intricate are its reactions. These reactions are 
those of its constituent units, and the cell is a convenient 
and necessary unit of structure. The cell is likewise an im- 
portant physiological unit, and as such requires special con- 
sideration. 

Reference Books and Texts l 

Bailey, L. H. Cyclopedia of American Agriculture. 1 : 618 pp., 

756 figs., 25 ph., 1907 ; 2 : 699 pp., 907 figs., 19 ph., 1907. 
Barnes, C. R. Physiology (in Coulter, Barnes, and Cowles, 

College Botany, Part II), pp. 297-484, figs. 619-699, 1910. 
Clements, F. E. Research Methods in Ecology. 334 pp., 85 

figs., 1905. 

Plant Physiology and Ecology. 315 pp., 125 figs., 1907. 

Curtis, C. C. Nature and Development of Plants. 471 pp., 

342 figs., 1907. 
Darwin, F., and Acton, E. H. Practical Physiology of Plants. 

321 pp., 43 .fas., 1894. 
Detmer, W. Practical Plant Physiology. (Transl. by S. A. 

Moor.) 555 pp., 118 .fas., 1898. 

1 In this list of reference works it is intended to include some of the 
more useful texts and general works which contain physiological infor- 
mation along many lines. Other books of greater specialization are in- 
cluded under the selected references for particular topics. 



Introduction 13 

Detmer, W. Das kleine Pflanzenphysiologische Prakticum. 

(3d Ed.) 319 pp., 179 figs., 1909. 
Ganong, W. F. Plant Physiology. (2d Ed.) 265 pp., 65 figs., 

1908. 

The Teaching Botanist, 439 pp., 40 figs., 1910. 

Goodale, G. L. Physiological Botany. 533 pp., 214 figs., 1885. 
Green, J. R. An Introduction to Vegetable Physiology. 459 

pp., 182 figs., 1907. 
Haberlandt, G. Physiologische Pflanzenanatomie. (4th Ed.) 

650 pp., 291 figs., 1909. 
Hansen, A. Pflanzenphysiologie. Die Lebenserscheinungen 

und Lebensbedingungen der Pflanzen. 314 pp., 160 figs., 

1898. 
Johnson, S. W. How Crops Feed. 375 pp., 1904. 
Jost, L. Plant Physiology. (Transl. by R. J. H. Gibson.) 564 

pp., 172 figs., 1907. 
MacDougal, D. T. Practical Text-book of Plant Physiology. 

352 pp., 159 figs., 1901. 
Osterhout, W. J. V. • Experiments with Plants. 492 pp., 252 

figs., 1905. 
Peirce, G. J. A Text-book of Plant Physiology. 291 pp., 23 

figs., 1903. 
Pfeffer, W. Physiology of Plants. (Transl. by A. J. Ewart.) 

1 : 632 pp., 67 figs., 1900 ; 2 : 296 pp., 31 figs., 1903 ; 3 : 451 

pp., 70 figs., 1906. 
Sachs, J. Lectures on the Physiology of Plants. (Transl. by 

H. M. Ward.) 836 pp., 455 figs., 1887. 
Schimper, A. F. W. Plant Geography on a Physiological Basis. 

(Transl. by Groom and Balfour.) 839 pp., 502 figs., 4 

maps, 1903. 
Sorauer, P. A Popular Treatise on the Physiology of Plants. 

(Transl. by F. E. Weiss.) 256 pp., 33 figs., 1895. 
Stevens, W. C. Plant Anatomy. 349 pp., 136 figs., 1907. 
Strasburger, Noll, Schenk, and Karsten. A Text-book of 

Botany. (Transl. by W. H. Lang.) 746 pp., 779 figs., 

1908. (Later German Ed. by Strasburger, Jost, Schenk, 

and Karsten.) 



14 Plant Physiology 

Verworn, Max. General Physiology. (Transl. by F. S. Lee, 

from 2d German Ed. ; 5th German Ed., 1909.) 615 pp., 

285 figs., 1899. 
Vines, S. H. Lectures on the Physiology of Plants. 710 pp., 

76 figs., 1886. 
A Student's Text-book of Botany. 1 : 430 pp., 279 figs., 

1895 ; 2 : 431-821, 483 figs., 1895. 
Warming, E. Ecology of Plants. (Transl. by Percy Groom 

and Isaac Bayley Balfour.) 422 pp., 1909. 



CHAPTER II 
THE PLANT CELL 

Certain aspects of the physiology of complex organisms 
may be convincingly presented and perhaps adequately 
understood without necessarily assuming any knowledge 
of the minute structure of such organisms. In the same 
way the demonstration of important chemical facts and 
reactions may be measurably feasible and instructive 
for those with little or no conception of the significance of 
atoms and molecules. Nevertheless, in the same way that 
a knowledge of the atom is indispensable in understanding 
chemical theory, just so the minute structure and the re- 
lations of cells is fundamental in order to gain a compre- 
hensive view of the activities of a multicellular organism. 

A century ago it became apparent to a few physiologists 
that some fundamental physiological problems could find 
more nearly complete solution only through experimental 
studies upon the cell. In the time which has since elapsed 
the relative importance of cell physiolog}^ has been more 
and more appreciated. Advances in this field, however, 
are necessarily associated with advances in morphology, 
chemistry, and physics. The development of cell mor- 
phology has been dependent largely upon the improvement 
of the microscope, and of current methods of technique, 
both of which have now reached a high state of per- 
fection. Physical and chemical theory and method have 
15 



16 Plant Physiology 

undergone profound changes, and the method of these 
sciences is now the method applicable to a study of all 
matter. In view, then, of the relationship of cell physi- 
ology to morphology, on the one hand, and to physico- 
chemical advances, on the other, a study of the cell has 
become fundamental for any comprehensive view of gen- 
eral physiology. 

9. The cell a physiological unit. — Representing the pro- 
toplasmic unit, the living cell is ultimately the seat of all 
those complex chemical and physical changes, or diverse 
energy transformations, of the living body. As a unicel- 
lular organism the cell must act independently in a par- 
ticular response ; in a multicellular body it responds also 
as a distinct unit, in unison, however, with many other cells 
associated together as a tissue. In any case it has been by 
an investigation of the cell that many of the principles of 
absorption, digestion, assimilation, excretion, and respira- 
tion have been demonstrated. The fundamental concep- 
tions of growth and differentiation, of fertilization and 
reproduction, were only possible through the development 
of cell study. 

Every cell passes through a cycle of changes. Each is 
a seat of many, if not of all, of the physiological processes 
characterizing the organism as a whole. In the more 
complex plants and animals diversity of labor among the 
cells has developed to such an extent that certain cells are 
restricted, or specialized, with respect to their activities, 
but all cells must perform certain fundamental functions 
necessary to growth, development, and differentiation. 

In almost any physiological process, or in the ultimate 
effects of various stimuli upon the organism, the cell is 



The Plant Cell 17 

" the important substratum of all vital activity." Refer- 
ring to the cell-theory and the importance of it, Wilson 
concludes: " No other biological generalization, save only 
the theory of organic evolution, has brought so many ap- 
parently diverse phenomena under a common point of view 
or has accomplished more for the unification of knowledge. 
The cell-theory must therefore be placed beside the evolu- 
tion-theory as one of the foundation stones of modern 
biology." 

10. Early use of the term " cell." — In the earlier studies 
upon the cell, beginning in the latter part of the seven- 
teenth century, the term was applied to the firm walls 
alone, from their resemblance to the cells of the honey-comb. 
When, however, protoplasm, or the living substance within, 
was later discovered, and its significance as the important 
morphological and physiological unit determined, the 
same term was retained for this essential unit of living 
substance. Nevertheless, with the obvious distinction in 
mind, the term is still applied to the many cell-forms or 
cell-cavities, from which all living matter has disappeared, 
— such cell-forms of many types constituting the great 
bulk of the conductive tissues of woody plants, and all 
of the heart-wood, stony tissue, dry bark, and the like. 

11. Meristem or embryonic cells. — Structurally or 
physiologically the term "cell" is now employed to denote 
the simplest unit into which the organism may be con- 
veniently resolved. It consists essentially of a unit mass 
of living protoplasm with certain inclusions or surround- 
ing materials. 

In plants the protoplasm is usually inclosed by firm 
often box-like cell-walls. Plant cells are usually so diverse 



18 



Plant Physiology 



that it is difficult accurately to speak of a typical cell. 
Nevertheless, in the higher plants, those cells which make up 
the meristem, or growing tissues, possess certain character- 
istics in common, and they may be considered typical in 
this restricted sense. All other tissues are derived from 

the meristem, hence its 
peculiar importance. 

A vegetative cell of 
the growing root apex 
of corn (Fig. 3) is 
more or less isodia- 
metric in form, often 
shown as a rectangle 
or polygon in section. 
The granular proto- 
plast, or protoplasmic 
body, differentiated, as 
denoted later, may be 
distinguished in such 
cases with compara- 
tive ease. It is closely 
surrounded by the 
firm cell-wall which is 
in general more con- 
spicuous and serves 
better than the protoplast to differentiate the limits of the 
cell units. The protoplasm is further differentiated into a 
dense, often rather centrally disposed, spheroidal body, 
the nucleus, and a less dense but granular enveloping 
cytoplasm. In the cytoplasm, when the meristem is 
included within the green tissues, there may also be noted 




3. Cell of the meristem, from root 
apex of corn. 



The Plant Cell 19 

certain small refractive protoplasmic bodies termed 
chromatophores. Vacuoles and food-materials of various 
sorts may also occur as inclusions within the general 
protoplast. 

An examination of rectangular or polyhedral cells at a 
short distance back of the tip will reveal certain changes, 
often denoting a passage from the formative to the non- 
formative or adult type. In the latter the cell is larger, 
the cytoplasm less abundant, and much of the cell-cavity 
may become occupied by vacuoles filled with cell-sap. 
As the vacuoles form the cell may show radiate or strand- 
like cytoplasmic areas usually connecting the central with 
the peripheral cytoplasm, but the nucleus may be still 
more or less central, and is invariably imbedded in cyto- 
plasm. As the change goes on the sap cavity enlarges and 
all the protoplasm is drawn to the periphery, the nucleus 
occupying the center of a marginal mass. 

The preceding defines the general type of many of the 
living cells of the plant body not undergoing rapid growth 
and division. In the active parenchyma of certain roots, 
tubers, and other organs to which there has fallen the 
office of starch or other food-stuff accumulation, the cell 
may become packed with such products ; still, nucleus 
and a thin layer of protoplasm remain, and these are later 
essential in the digestion and transport of the stored food- 
materials. 

12. Cytoplasm. — When reference is made to the struc- 
ture of protoplasm, it is usually the cytoplasm which is 
considered. Fixing the attention upon the cytoplasm 
alone, it is found that this feature of meristematic cells, 
of germinating pollen grains, of the hyphse of black-mold 



20 Plant Physiology 

fungi, of slime molds, — indeed of all plants, — is much 
alike. The cytoplasm is evidently a semiliquid trans- 
lucent substance, and it contains granules, resulting usually 
in a distinctly granular appearance. All protoplasm is 
readily killed by a solution of iodine by which it is also 
stained yellowish brown. This reagent is therefore con- 
venient in demonstrating protoplasm in cells where it is 
not readily visible. Moreover, the use of a strong salt 
solution, causing contraction of the cytoplasm from the 
cell- wall, is also important in demonstration. The outer 
margin of the cytoplasm, or the margin bordering a vacu- 
ole, possesses important physiological characters, and for 
convenience it is called the plasma membrane. The 
cytoplasm may inclose food-materials not easily distin- 
guished from the usual protoplasmic granules. 

The structure of protoplasm has received much con- 
sideration, and three noteworthy conceptions of its form 
have been advanced as follows : (1) the netted or reticu- 
lum theory; (2) the fibrillar theory, and (3) the alveolar 
or foam theory. The present tendency is to regard it as 
unnecessary to assign a definite structure persistent under 
all conditions, and physiologically it is logical to believe 
that the structure is simply a manifestation of a type of 
activity. Stains of various sorts have been most im- 
portant in the study of cytoplasm as well as of nucleus. 
The chemistry, movement, and special responses of pro- 
toplasm in general are considered later. 

13. The nucleus. — The nucleus presents the appear- 
ance of a dense or refractive protoplasmic mass, and it 
does contain denser bodies. It is often nearly spherical, 
but in certain old or specialized cells it may be irregular 



The Plant Cell 21 

in form. It is differentiated from the cytoplasm by a 
distinct membrane, but the minute structure is only ap- 
parent by the use of staining methods. It shows usually 
a strong affinity for many stains, and its parts may react 
in a differential manner. In the growing nucleus there is 
usually a refractive reticulum staining rather lightly in 
general, but deeply at certain points, or angles, where there 
is or seems to be an aggregation of chromatic substance. 
There is also present upon the reticulum at least one 
nucleolus. This latter is most evident by staining, but 
in the unstained nucleus it is strongly refractive, and often 
serves to locate the nucleus. 

Nucleus and cytoplasm are interdependent, and few 
cells are long functional in which either of these parts is 
killed, or from which either is removed. 

14. Plastids. — In addition to cytoplasm and nucleus 
the other protoplasmic organs of the cell requiring brief 
mention at this point are dense bodies termed plastids, 
usually disposed in the parietal protoplasm of the cell. 
In the higher plants they are usually spheroidal or ellip- 
soidal in form. Of these there are three types : (1) chlo- 
roplasts, containing the pigment chlorophyll, to which is 
due the green color of plants, essential, as shown later, 
in the manufacture of organic food-material in the green 
plant ; (2) leucoplasts or amyloplasts, those starch-form- 
ing plastids contained in subterranean or other organs of 
the plant receiving no light, — plastids, nevertheless, 
which are able, when exposed to light, to develop into chlo- 
roplasts ; and (3) chromoplasts, plastids of various colors, 
generally yellowish to red, sometimes crystalline in form, 
from the presence of albuminous crystals, or from the crys- 



22 Plant Physiology 

tallization of the pigment contained. The special signifi- 
cance of these three types of bodies will require treatment 
later. 

15. The cell-wall. — The plant protoplast is commonly, 
and in the vegetative organs of higher plants invariably, 
invested by a firm cell-wall. When constituting a part 
of a tissue-system, the cell-walls are throughout most of 
their length in close contact, mutually supporting, and, 
with the modifications subsequently noted, they form 
together a complex circumcellular organic skeleton. Some 
walls are also infiltrated with mineral matters; espe- 
cially are the outer walls of grasses and the like silicified. 

The form of the cell-wall is, of course, in all cases a per- 
fect index of the form of the cell, although in some cases 
after the death of the protoplasm the ceil may be some- 
what modified in shape. The wall is formed by the pro- 
toplasm and it is properly regarded as a product of pro- 
toplasmic metabolism or secretion. It is commonly 
composed of two or three distinct layers, three often occur- 
ring when the wall is strongly thickened. In special cases 
where successive layers are deposited, the wall may present 
a laminated structure. In the formation of the cell-wall 
in general a middle lamella is first laid down. This is the 
primary layer, and upon it is deposited a secondary, and 
finally a tertiary, layer. The second layer is, as a rule, 
the thickest or most completely developed. In the looser 
tissues of the body the middle lamella may split at the 
angles between the cells, thus leaving intercellular spaces 
of greater or less extent, the importance of which in gas 
diffusion through the plant will subsequently receive 
special consideration. 



The Plant Cell 23 

The successive layers in the formation of the cell-wall 
may be interrupted at certain points or along certain lines, 
and there will thus result pores or pits of various types. 
Again, the thickening may be confined to particular re- 
gions, so that the peculiarities of the wall may be consid- 
ered due to the deposition of the layers in very limited 
areas, as in the annular or spiral vessels. In the tracheal 
tissue these pores may occur in adjacent cells opposite to 
one another, so that the cells at these points are in reality 
separated merely by the primary layer. Such connections 
are important in the transport of water and substances in 
solution; but it is not the province of the present brief 
description to make an examination of wall structure, 
the aim being merely to indicate the mechanism of special 
physiological interest. 

In the case of the soft-rot of cabbage and other vege- 
tables, the causal bacillus attacks and decomposes the 
middle lamellae, so that the organization of the tissues is 
promptly broken down. Gelatinization also of the cell- 
wall may occur in seed-coats. Flax, mistletoe, and some 
other plants exhibit this phenomenon when placed under 
conditions favorable for germination. 

16. Cell-sap. — The protoplasm is infiltrated with water, 
and there are closely associated with it nutrient and other 
substances in solution. Moreover, as already indicated, 
there are generally present within the protoplasm some 
definite " vacuoles," also containing substances in solu- 
tion ; and such solutions are called cell-sap. These vacu- 
oles are apparently of much the same nature as the large 
central one ultimately formed in the great majority of 
differentiated cells. The term " cell-sap," at any rate, is 



24 Plant Physiology 

indiscriminately applied to the liquid contents occurring 
in the vacuolate areas large and small. 

The vacuoles are unquestionably of much physiological 
significance, and certain materials diffuse into such spaces 
and may to a considerable extent accumulate there. 
The substances held in solution may include a variety of 
organic or inorganic compounds, again referred to in the 
discussion of metabolic products. In some cases the color 
of plants is due to coloring matters occurring in the cell- 
sap alone. With respect to the other liquid cell contents 
the vacuoles may have therefore a certain differential 
character. 

17. Cell-forms. — Young or growing cells in tissues are 
often somewhat rectangular in outline. When, however, 
the pressure of adjacent units is released, there is an ap- 
parent tendency to assume a form more or less spherical. 
This should not be confused with the fact that micro- 
organisms possess a considerable diversity with respect to 
their specific forms ; indeed many unicellular organisms 
of elongate forms which grow for a time in pairs or groups 
become also more convex along the lines of attachment 
upon being set free ; thus rod-shaped bacteria may become 
more rounded at the ends. In the meristem of the grow- 
ing tip'where the cells are closely united, and encompassed 
by a variety of pressures, the typical form of the cell is 
isodiametric or polyhedral. Back of the formative region, 
under the influence of the growth pressures and various 
other stimuli, there is a tendency for many cells or cell- 
groups to take up an elongate form. The latter may 
make possible, in time, other important modifications. 

Under all circumstances the embryonic meristem cell 



The Plant Cell 



25 



is capable of changing its form or of undergoing differen- 
tiation when in a position where this response is called 
forth. It must be un- 
derstood that this dif- 
ferentiation is in direct 
or indirect response to 
a variety of stimuli 
which are normally 
operative during the 
growth of the plant. 
It is found, therefore, 
that while all the cells 
of higher plants have 
developed from an 
original meristem of 
the type indicated, 
there is the greatest 
diversity in the ulti- 
mate form, as also in 
the ultimate work, of 
the cells differentiated 
therefrom. 

The different types 
are, of course, associ- 
ated with a specialized 
form of labor, or func- 
tion ; therefore these cell-types are of peculiar physio- 
logical importance, as well as of obvious anatomical and 
evolutionary interest. The following common types may 
be briefly characterized for further reference : — 

18. Parenchyma. — This t^^pe includes various forms 




Fig. 4. Cell from a leaf-hair of squash, 
showing vacuolate cytoplasm, nucleus, 
and chloroplasts. 



26 Plant Physiology 

of relatively thin-walled cells which may have undergone 
very little, though sometimes considerable, change in 
shape. They are generally more or less rectangular or 
polygonal in outline, and frequently exhibit large inter- 
cellular spaces. In some cases, especially when the proto- 
plasm has been lost, the walls may be infiltrated with 
mineral matters. In situations where they ma}^ be 
directly or indirectly exposed to the drying action of the 
air, the walls may contain cutin or suberin, thus rendering 
them less penetrable to the passage of water. If the 
walls are thickened at the angles, as in the supporting 
cells of the cortex, they are commonly termed collen- 
chyma. 

Parenchyma of some form almost invariably accom- 
panies conductive tissues, but it is not particularly adapted 
for the rapid movement of solutions, being in large part 
dependent upon simple diffusion phenomena. It has been 
found, however, that in the parenchyma there are com- 
monly minute cytoplasmic connections between adjacent 
protoplasts; that is to say, minute pores may occur in 
walls separating cells, and through these pores cyto- 
plasmic fibrils may extend, connecting therefore adjacent 
protoplasts. These connections may be of great impor- 
tance in the relations existing between the cells in paren- 
chymatous tissue. 

19. Sclerenchyma. — This term is usually employed to 
denote cells with considerably thickened walls. Thick- 
ening may proceed to such extent that the protoplast dis- 
appears and the lumen may be practically closed. The 
grit cells of the fruit of pear are much thickened, and the 
stone cells of the " pits " of drupaceous fruits, or of the 



The Plant Cell 



27 



shells of nuts, are extreme examples. Elongate cells with 
thickened walls may also be included here, such as bast, 
or those surrounding the bundles in Indian corn (Fig. 5). 




I 



B 




Fig. 5. Some extreme cell-types : collenchyma (a) ; sclerotic cells from 
pine needle (6) ; and vegetable ivory (d) ; unusual palisade cell (c) ; bast 
(e, /) ; prosenchyma (g) ; from cotton seed-coat (h) ; and stinging cell of 
nettle hair (t). 



The term " stereome " has been applied to thick-walled 
cells serving primarily for support. 

20. Tracheids. — These are thick-walled cells, more or 
less elongate, with walls often showing pitted, reticulate, 
or spiral thickenings. They may possess a considerable 
lumen, and the mature cell may show no trace of proto- 
plast. They are usually lignified, and are important in 
the conduction of water. In many plants (such as the 
pine and other conifers) they constitute the sole water- 
conducting system. 

These tissues are likewise important in support, and this 
fact emphasizes a point worthy of special note, and that is 



28 Plant Physiology 

this : a tissue may be primarily important for a specific 
type of action, but differentiation is not commonly so 
complete as to render it unserviceable in many correlated 
activities. 

21. Tracheae or vessels. — The vessels are formed 
from rows of elongate cells by the absorption of the inter- 
vening walls coincident with the disappearance of the 
protoplasm. Such cell-cavities, or ducts, may extend 
continuously for several centimeters in length, and they 
are especially important in the conduction of water in 
angiosperms generally. These ducts also show usually 
the annular or spiral thickenings previously referred to. 
Both this and the preceding type are commonly associated 
with at least a small amount of parenchyma, and it is 
probable that their physiological properties depend to 
some extent upon the latter. 

22. Sieve tubes. — The sieve tubes are also elongate 
cells, but they are peculiar in the fact that the protoplasm 
in the adjacent members of the cell-row is continuous by 
means of very distinct connecting pores through the 
intervening walls. These walls are thickened and form 
a so-called cell-plate, a perforate plate through which, 
therefore, the protoplasm is continuous (Fig. 6). Cell- 
plates may also occur at points of contact in more or less 
vertical walls. 

Another peculiar feature of such sieve cells is the fact 
that the nuclei become disorganized, but the cytoplasm 
remains. These cells, however, are in close contact with 
certain cells which are typical parenchyma elements, the 
companion cells, the latter containing both cytoplasm 
and nuclei. The sieve tubes usually occur in the woody 



The Plant Cell 



29 



bundles, and are easily identified in the outer part of 
these (commonly in the bark, therefore, of dicotyledonous 
plants). They are regarded as most important in the 
conduction of the less diffusible organic materials. 

The arrangement or association of certain of these types 
of cells and further indications respecting their several 
functions in the general physiology of the plant are again 
referred to under growth and transport. 




Fig. 6. Conducting cells of fibrovascular bundles, ducts, tracheae (I) ; 
pitted vessel (m) ; sieve tubes with companion cells, in longitudinal (n) 
and cross section (o). [Adapted.] 



23. Protoplasmic movement. — Naked protoplasmic 
units or aggregates such as amoebae, or the plasmodia of 
Myxomycetes, show a considerable power of locomotion 
due to streaming movements in the cytoplasm. It is 
also well established that within the protoplast of a variety 
of cells invested with a firm cell-wall there is relatively 



30 Plant Physiology 

rapid movement. It is a phenomenon so common l 
that it must be assumed to have physiological significance. 
Moreover, it is often rapid in cells of large size, so that 
it seems safe to say that it is not unimportant in facili- 
tating diffusion. A study of this capacity for movement 
gives an impressive mental picture of the protoplasm as 
the seat of activity in the cell or member. 

The various types of protoplasmic movement are com- 
monly grouped in four categories : (1) simple streaming, 
(2) circulation, (3) rotation, and (4) orientation. 

Streaming movements are rather spasmodic in different 
parts of the cell, first in one direction, and sooner or later 
a reversal. Aside from the slime molds, the coenocytic 
filaments of the black molds show, among plants, pro- 
nounced movements of this type. 

Circulation consists in movement at any instant in 
more than one direction in the cell. The motion may 
occur in the peripheral cytoplasm, but this type is char- 
acteristic of cells possessing cytoplasmic strands. In 
fact, when the cytoplasmic strands are lost, the movement 
may become rotary. Circulation may be conveniently 
observed in the stamen hairs of Tradescantia and in the 
stem and leaf hairs of various plants, especially cucurbits; 
also in young root-hairs and other young cells. 

Rotation, or the movement in a rather constant current 
or direction around the cell, or in some area of the cell, is 
the most striking type. It usually occurs in cells which 

1 For a list of greenhouse material suitable for the study of movement 
the following paper may be consulted : Bushee, Grace L., The Occur- 
rence and Rate of Protoplasmic Streaming in Greenhouse Plants. Botan. 
Gaz., 46:50-53, 1908. 



The Plant Cell 31 

have lost the cytoplasmic strands. It may be observed 
in the water weed Elodea, and attains a maximum rate 
and clearness in an inner parietal cytoplasmic layer of 
the internodal and other ccenocytic segments of the alga 
Nitella (stonewort). In this last-named plant it is not 
uncommon to find, at a temperature of 28 to 30° C, a 
rate of movement from 3 to 4 mm. per minute. 

Movements of orientation result in a gradual, or scarcely 
directly visible, shifting of a portion of the cytoplasm or of 
other portions of the protoplast. By this means the nu- 
cleus is able to change its position in the cell, and the plas- 
tids (chloroplasts) show peculiarities of arrangement under 
different intensities of light. Orientation is doubtless to 
a considerable extent characteristic of all living cells, but 
the result of the movement is more easily noted in those 
green cells quickly responsive to changes of light. 

24. Protoplasmic irritability and response. — The move- 
ments previously referred to are indicative of a type of 
activity. It is of interest to note to what extent this 
activity may be affected by a change in the environment, 
as, for instance, a change in temperature. If favorable 
material of any kind (such as Nitella, Tradescantia, 
Cucurbita) is carefully studied at different temperatures, 
effected by a temperature stage, it will be found that with 
respect to rate of movement there is, in general, a mini- 
mum, an optimum, and a maximum temperature for move- 
ment, so that the protoplasm is highly responsive to these 
differences of the environment. 

This change in rate of motion with the above-mentioned 
minimum-optimum-maximum manifestation is obviously 
an indirect effect. Moreover, since temperature changes 



32 Plant Physiology 

are a constant environmental factor, it is safe to assume 
that the organism is adjusted to this factor, so that it 
manifests what is known as a tonic response. 

Another case of response has already been noted : under 
different intensities of light the orientation of the chloro- 
plasts may be diverse. In the cells of the duckweed 
(favorable for observation) the chloroplasts are distributed 
in the upper portion, or dome, of the cell and also across 
i the bottom, in diffused light ; while in bright light they 
lie at the sides and one above another. The position in 
the dark is along the vertical walls, also the horizontal 
wall when that does not abut upon the epidermis. This 
type of response to light (in this instance) is generally 
regarded as denoting protoplasmic irritability. 



LABORATORY WORK 

Living cells. — Remove with the forceps or scissors the fila- 
mentous, purplish hairs from the stamens of any available 
species of Tradescantia. Mount these, and note carefully the 
form and size of the distinct cells. Distinguish cell-wall, pro- 
toplasm, and colored vacuole, and observe each of these crit- 
ically. Describe the peripheral and strand cytoplasm, also the 
form and position of the nucleus, with nucleolus. Draw. Com- 
pare the cell drawn with others both nearer the base and the 
apex of the filament. Kill with tincture of .iodine, and examine. 

As in the preceding, study the cells of hairs clipped from a 
petiole of a fairly young squash or pumpkin leaf. In this case 
note also the form and position of the plastids (chloroplasts). 
Peel off a little of the epidermis of Cyclamen or Begonia ; 
mount, study, and describe these cells. For comparison study 
and draw a stained preparation of a root-tip or bud-apex and 
compare with the previous material. 



The Plant Cell 33 

Cell-forms. 1 — Study parenchyma in the young stem of 
Indian corn, or in the pith of an herbaceous plant, also collen- 
chyma in the peripheral portion of a stem of wild carrot or 
squash. Sclerotic cells may be easily identified in the gritty 
portion of the fruit of pear. Tracheids and tracheae may be 
studied by means of longitudinal sections of almost any woody 
plant, grape vine and squash showing, particularly, ducts of 
considerable size. These plants are also good for an examina- 
tion of sieve tubes. 

Cells such as the tracheids may be conveniently studied 
after maceration. Place sections of the desired material in very 
strong or concentrated chromic acid for about one minute, or 
until thoroughly limp and easily teased apart ; then wash and 
tease out, or mount immediately, and separate the cells one 
from another by gentle pressure upon the cover-glass. 

Protoplasmic movement. — Following the general indications 
in the text, study types of protoplasmic movement in the cells 
of such plants as Nitella and Elodea ; hairs of Tradescantia, 
of a cucurbit, or of Gloxinia ; and the young hyphae of any 
common black mold, Mucor. 

Use the most favorable material for further observation upon 
the effects of temperature upon movement, employing a tem- 
perature stage in determining the rate of movement at tem- 
peratures varying from towards the minimum to an approximate 
maximum. Plot a curve of the results. 

References 

Butschli, O. (Eng. Ed., transl. by E. A. Minchin.) Investi- 
gations on Microscopic Foams and on Protoplasm. 379 
pp., 23 figs., 12 pis., 1894. 

1 It is considered important that students not qualified in anatomy or 
histology should devote several laboratory periods to a study of the cell 
and cell-forms. Reports based upon their own observations may be 
supplemented by a more complete review of the subject as presented in 
Stevens, Strasburger, Vines ("Text-book of Botany"), or other suitable 
text. 



34 Plant Physiology 

Ewart, A. J. On the Physics and Physiology of Protoplasmic 

Streaming in Plants. 131 pp., 17 figs., 1903. 
Fischer, A. Fixierung Farbung und Bau des Protoplasmas. 

362 pp., 21 figs., 1 pi., 1899. 
Hertwig, O. The CeU. 368 pp., 168 .fas., 1895. 
Loeb, J. The Dynamics of Living Matter. 233 pp., 64 figs., 

1906. 
Wilson, E. B. The Cell in Development and Inheritance. 

(2d Ed.) 483 pp., 194 figs., 1906. 

Texts. Vcrworn, Jost, Pfeffer, Ganong, Strasburgcr. 



CHAPTER III 

THE WATER-CONTENT OF PLANTS AND 
THE GENERAL RELATIONS OF ROOT 
SYSTEMS 

The life and special activities of the plant or animal 
are at all times conditioned by the water-supply. Plant 
growth and production may be more sharply limited 
within countries, regions, or localities by the water-supply 
than by any other factor of the ordinary physical environ- 
ment. A soil which does not receive and deliver to the 
plant throughout the growing season a reasonably constant 
supply is a sterile desert whatever may be the quality of 
this soil with respect to latent mineral possibilities. 

Water is often regarded as a crude food-stuff, because 
it enters abundantly into the composition of living things. 
It does, in fact, contribute elements to the making of or- 
ganic food, as shown later ; but for the moment it is most 
important to consider water with respect to its solvent 
action. All organic food-material presented to the living 
cell must be in solution; likewise the mineral nutrients 
and the gases which take part in metabolism. Ordinary 
plants are constantly in contact with a water-supply, 
during their growing period, by means of special absorb- 
ing surfaces. It is to be expected, therefore, that the 
forms and functions of plants are to a considerable degree 
35 



36 



Plant Physiology 



concerned with the use and distribution of water and of 
substances in solution. 

Properly to consider the use of water there arises the 
necessity of learning or reviewing the structure of the 
organs and the nature of the processes whereby water is 
absorbed, conducted, and eliminated, as well as general 




Fig. 7. Thrifty squash-plants as typical examples of the relation of 
water-content to rigidity. 

and special crop relations in which this factor plays an 
important role. 

25. Hydrostatic rigidity. — Under conditions favorable 
for growth, it is obvious that the living cells of a plant are 
commonly in a state of extension or hydrostatic rigidity. 
Small and succulent stems are able under such circum- 



Water-Content of Plants 37 

stances to support a considerable load of branches, leaves, 
and flowers. Any condition which deprives the plant of 
water inaugurates, on the contrary, a state of flaccidity; 
that is, a drooping or wilting. These phenomena will be 
referred to again, but the fact may not be too strongly 
emphasized that rigidity and abundant water-supply are 
closely related, especially where the mechanical supporting 
tissues do not reach the fullest development. Compare 
the appearance and vigorous yield of well-watered lettuce, 
squash, or tomatoes with those unattractive and miser- 
able plants whose leaves or fruits fall limp upon them- 
selves. 

26. The water-content of plants. — The growing plant 
contains invariably a high percentage of water. It is gen- 
erally stated that an active, succulent plant or tissue, one 
which contains relatively a small amount of fiber, shows 
a water-content of 75 per cent or more. When the plant 
contains a larger number of thick-walled cells, or woody 
tissues, which may be required for protection, support, 
or conduction, the percentage of water may be lessened. 
In every case, it is probable that the active protoplast 
requires a water-content of from 80 to 90 per cent or more. 
The necessary water must be obtained by absorption from 
the environment, and in the case of the common agri- 
cultural plants absorption is almost exclusively by means 
of the root-system. 

The amount of water contained in different plants, or, 
in fact, in the same plant, is subject to considerable varia- 
tion. Nevertheless, it is instructive to note the composi- 
tion of a number of crop or useful plants with respect to 
this factor. The following table will indicate approxi- 



38 



Plant Physiology 



mately the average water-content of a number of familiar 
plants or plant products : — 



Water- 

COXTEXT, 

Weight 
Per Cext 



Apples, fruit .... 
Beets, mangel wurzels . 

Beets, red 

Beets, sugar .... 

Beets, tops 

Cabbage 

Clover, red, green hay . 
Clover, white, green hay 
Corn, dry seed .... 
Corn fodder, green 

Corn silage 

Cowpeas, green hay . 



Cucumbers 

Oats, cured grain 

Onions 

Potatoes, Irish 

Potatoes, sweet 

Pumpkin, flesh 

Rice, grain 

Spruce needles, old, in 

tober .... 
Spruce needles, young : 

spring .... 
Timothy hay, cured 



o,.. 



96.0 
11.0 
87.6 
78.9 
71.1 
93.4 
12.6 

56.7 

80.6 
42.2 



27. Variation in water-content of different organs. — 
An examination of the various analyses reported by chem- 
ists will indicate that the different products or organs of 
the same plant may vary materially in the water-content, 
as would be anticipated. This may be due in part to 
differences in the amount of supporting or otherwise dif- 
ferentiated tissues. Fruits may, however, contain much 
more water than the growing shoots upon which they are 
developed, or certain fruits may contain at maturity very 
much less. This will all depend upon the nature of the 
tissues in these parts, and upon the degree of maturity, 
or the method by which maturity is accomplished. 

During the ripening of seeds the water-content may be 



Water-Content of Plants 39 

reduced by several hundred per cent. This may go on 
simultaneously with a reduction in the water-content of 
the plant as a whole, which is the case in cereals and many 
other plants having a definite growth cycle. On the other 
hand, the maturity of the seed in many annuals and per- 
ennials which grow in an indefinite manner may be wholly 
independent of any general ripening process of the entire 
plant. 

The seed within the body of the fruit may likewise differ 
from the latter ; thus the seed of watermelon or peach 
shows, when the fruit is ripe, a water-content far less than 
that of the pulp which surrounds it. The water of the 
plant does not merely permeate all parts indiscriminately ; 
it is accumulated within or withheld from organs b} r virtue 
of complex histological, chemical, or physical relations. 
The formation of a few layers of corky tissue may cut off 
the water-supply of an organ, the storage of solid food- 
materials may reduce the quantity of water, or the pres- 
ence of certain compounds may increase or inhibit absorp- 
tion. 

28. The water-absorbing system. — The root-system 
constitutes the mechanism whereb}^ the water-supply 
must be secured in practically all higher plants, including 
the common agricultural plants. There is, moreover, 
diversity in the form, texture, and distribution of the roots 
of crop plants. The diversity in form and texture is not 
necessarily coupled with great differences respecting the 
water-supply furnished. 

There are two general types of root complexes ordinarily 
recognized. In the one there may be a central or main 
root called the tap-root, the branches and sub-branches 



40 



Plant Physiology 



of which arise in rather irregular order, but make up a 
general root-system which may occupy a fusoidal, a coni- 
cal, or an obconical soil volume. This type includes let- 
tuce, parsnip, and a 
great variety of common 
plants. In the other 
type there may be little 
or no indication of a tap- 
root, and instead few or 
many lateral roots of 
more or less equal size 
may in a way take its 
place. Corn, for in- 
stance, possesses at the 
beginning of germination 
a distinct tap-root, but 
very soon, under ordi- 
nary circumstances, in 
the soil the length of this 
may be approached or 
exceeded by laterals or 
by whorls of secondary 
roots originating consid- 
erably later (Fig. 8). In 
many of the small cere- 
als there are produced 
upon germination several 
roots which may be termed of the first order, and the 
direction of growth of these determine for many days the 
general form of the system. 

It is a part of the function of the root-system to fix the 




Fig. 8. General appearance of the root 

system of corn at the time of tasseling. 



Water-Content of Plants 41 

plant in the soil, but of chief interest must be regarded the 
relations of the root-system to the water and the nutrient 
salts of the substratum. 

From a casual examination of plants in the field it is 
difficult to form a proper conception of the extent of the 
root-system. Pull up a wheat plant or any fibrous rooted 
grass, and the root-system may seem extensive. Proceed 
in the same manner with a beet or with an herbaceous 
plant like the sunflower. The root-systems which come 
to view in these two cases would give an entirely erroneous 
impression of the relative or actual extent of the roots. 
A large proportion of roots and rootlets remain in the soil, 
especially in the case of the fleshy plant. 

Upon a careful examination it is noted invariably that 
accompanying vigorous growth in the soil there is a sur- 
prisingly extensive system of small rootlets, and these are 
usually disregarded in rough estimates of root extent. The 
only methods of determining approximately the root devel- 
opment is either by excavating carefully, and then washing 
away the soil while the roots are in some way effectively 
supported, or by growing the plants in special root cham- 
bers. 

29. The rooting habits of crops. — Plants vary greatly 
with respect to the distribution of roots in the soil. In the 
same habitat, or under the same cultural conditions, one 
plant may show the greater extent of its roots close to the 
surface, while another may branch more freely at greater 
depth. Freidenfeld has made a careful study of root- 
habit in a variety of common plants. 

A study of this distribution of roots under diverse con- 
ditions is a matter of considerable importance. Upon it 



42 Plant Physiology 

may be based better practices in soil preparation and cul- 
ture. Investigations upon root distribution have been 
more extensive at some of the experiment stations in the 
West, and there are very few data available for conditions 
in the United States essentially different. Ten Eyck has 
shown that the roots of the corn, wheat, oats, and other 
cereals may reach a depth of from three to four and a half 
feet, the small grains reaching the greater depth. His 
method consists in supporting the cuboidal mass of soil 
containing the roots in wire-netting cages, through the 
meshes of which many steel rods are thrust horizontally. 
When the soil is washed away, the roots do not break so 
• readily as when unsupported. Nevertheless, this method 
has many difficulties and involves special apparatus for 
handling large quantities of earth. 

Recently a method has been developed in Russia by 
Rotmistrov whereby the difficulties experienced in han- 
dling large quantities of soil are to a considerable extent 
eliminated. Some new sources of error have been intro- 
duced, but apparently the work has been as well controlled 
as possible. The method consists in growing plants in the 
natural top-soil and sub-soil compacted into extensive 
narrow boxes sunken in the soil during the period of growth. 
When placed in position, these boxes present a surface 
1 inch wide, 20 to 40 inches long, and 20 to 40 inches or 
more in depth. The roots eventually occupy a volume 
of earth equivalent in form to that of a narrow slab or 
broad board. After the desired period of growth it is 
possible to obtain practically the exact form of the entire 
root-system by the manipulation suggested. Practically 
speaking, this consists in the inversion of the root-pene- 



Water-Content of Plants 43 

trated mass upon a support or screen closely studded with 
nails. Upon this screen careful washing is subsequently 
given, and the entire root-system is then readily transferred 
to cardboard. Figure 9 shows a root-system of the potato 
grown in this manner. About 30 plants were grown by 
Rotmistrov in such boxes for varying periods of time. 

As a control upon this method some plants were grown 
in natural soil beds, and the development and extension of 
the root-systems were studied by means of deep pits with 
horizontal chinks or tunnels. The pits permitted a care- 
ful record of the depth of root extension, and through the 
small horizontal chinks observation could be made upon 
lateral penetration. 

Working with the pit-ancl-chink method indicated, it 
was determined that even in seven days the roots of " a 
great many cultivated plants extend beyond the limits of 
the soil when tilled to a medium depth " (8 inches). The 
roots of winter grain often extend laterally and vertically 
to a distance of over 40 inches. Winter rye was found to 
extend to a greater depth, and winter wheat to a greater 
extent laterally, than other small grains. 

Among the roots of corn there may be distinguished, 
according to Ten Eyck, primary vertical and primary 
lateral organs. The latter in their course again give off 
vertical roots. The main laterals grow several feet, and 
as they reach those of neighboring hills, they also strike 
downward. Under the rather dry conditions of the West 
a majority of the lateral roots are within from three to 
twelve inches of the surface. When the sub-soil is poor, 
deep culture of corn may therefore kill the roots or prevent 
their formation in the most fertile parts of the soil. On 



44 



Plant Physiology 



the other hand, it is especially necessary under the dry 

conditions which 
prevail in that region 
to have a deep surface 
mulch. It is of inter- 
est to note that under 
these circumstances 
shallow cultivation 
early and deep culti- 
vation later has 
proved the most satis- 
factory method. 

Kafir corn and sor- 
ghum produce roots 
of the same general 
type and distribution 
as those of corn, but 
the former are tough 
and fibrous and The 
laterals arc beset with 
numerous, fine, feed- 
ing roots which are 
to be found between 
the main Laterals and 
the surface. The up- 
per eighteen inches of 
soil is very completely 
filled with these fine 
roots. Moreover, the 
Fig 9. Root-system of a potato grown to r^anf grows late in 
maturity in a deep, narrow box. [After 

Rotmistrov.j the season, and the 




Water-Content of Plants 45 

mere presence of the numerous tough roots and crowns 
are sufficient to leave the soil in bad physical condition. 
This unfavorable condition restricts the absorption of 
water by the soil during fall and winter, and discourages 
the requisite preparation for the succeeding crop. Owing 
to these facts, sorghum land commonly shows a defi- 
ciency in moisture the following spring. 

The rooting habit of the sugar-beet, according to Ten 
Eyck, indicates that it is a deep " feeder " at least during 
the late stages of growth. The roots of potato seem to 
occupy the soil as completely as any crop, and a consider- 
able number penetrate to a depth of two or three feet; 
yet many of the deep-rooted individuals possess a surpris- 
ingly small number of feeding roots, at least on plants 
examined in the autumn. 

Nobbe measured the root-system of a wheat plant about 
one year old and found the aggregate length of the roots to 
be 500-600 meters (545-655 yards), while that of a full- 
grown pumpkin vine measured about 50 times as long, 
or about 25 kilometers (15f miles). The rooting habits 
of shade trees are particularly worthy of study, especially 
in view of the difficulties experienced with trees in towns 
and cities. 

From the studies in narrow boxes made by Rotmi- 
strov it has been shown that a large number of farm 
crops penetrate both loamy and sandy soils to a depth of 
one meter or more. 

30. The production of root-hairs. — The roots and mi- 
nute rootlets which in a complete root-system are read- 
ily evident to the eye are, however, secondary with respect 
to the relations existing between the plant and the soil 



46 



Plant Physiology 



water, or the soil solution. When the plant is removed 
from the soil, even most carefully, organs smaller than 
the rootlets are. not made evident. 
There are, nevertheless, 'numerous 
minute, simple, and effective struc- 
tures generally present in abun- 
dance. These are the root-hairs 
arising from the surfaces of all 
young and growing roots. 

If seeds of radish or squash are 
placed in damp moss or germinated 
between sheets of moist filter paper, 
or in any of the germinators sub- 
sequently described, the root-hairs 
become evident (Fig. 10). As soon 
as the root has attained a length of 
an inch or more there are developed 
at a short distance behind the tips 
a large number of these structures. 
They arise practically perpendicular 
to the surface, and a microscopic 
Fig. 10. Radish seedling, examination indicates that they are 
showing root-hairs. ^^ siphonaceous cellg consisting 

of a rather resistant cell-wall within which is contained 
the granular protoplasm and cell-sap. 

When grown in the manner indicated, the root-hairs 
may be perfectly straight tubes. As they develop in the 
soil, however, where the numerous sharp soil particles 
obstruct their growth, they bend about and flatten out 
against and around these particles, becoming, as a result, 
contorted or deformed in appearance. It is evident that 




Water-Content of Plants 



47 



they come into the most intimate contact with the minute 
soil particles, — so intimate, at times, that fine particles 
actually stick into the walls (Fig. 11). They are, there- 
fore, peculiarly fitted for the needs of absorption, as will 
be later developed. 

It will also be noted that those regions of the rootlet 
clothed with root-hairs have ceased to elongate ; that is, 
so soon as the hairs are developed it is an indication that 
this portion of the root is fixed in the soil ; otherwise its 
growth would crush such organs and 
prevent their further efficiency. In 
this connection it may, therefore, be 
observed that the " push " which is 
needed to force the root forward in the 
soil is concerned with a relatively short 
axis, perhaps not more than a quarter 
of an inch in length. The practical ad- 
vantages of this mode of growth are 
obvious upon a moment's reflection. 
If, for instance, one should attempt to 
force into the soil a fine wire two feet 
long, pushing from the upper end, it 
would certainly bend. In fact, the 
difficulties of such a mode of growth 
in the soil practically precludes the pos- 
sibility of its occurrence. 

The root-hairs are relatively short- 
lived upon the majority of plants. 
Their activity may be embraced in a Fig. 11. Root-hairs 
period of from a few days to a month (a > f rown incoa ^e 

, . sand ; cortex (o) 

or two, and they are readily injured by an d epidermis (c). 




48 



Plant Physiology 



unfavorable conditions of the environment. The ability 
of a plant to take water from the soil, moreover, will de- 
pend in large measure upon the ex- 
tent of these simple organs. It has 
been estimated that the surface of 
the system in corn is increased from 
five to six times by favorable root-hair 
production; in barley about twelve 
times, and in Scindapsus about eight- 
een times. 

31. Root-hairs and the water-con- 
tent of the soil. — It is only in ex- 
ceptional cases that land plants de- 
velop few or no root-hairs. In general, 
where few are present, the plant will 
wilt with a higher water-content of 
the soil than when more are provided. 
Root-hairs are considerably suppressed 
in the case of corn, wheat, and other 
crop plants when the soil is saturated. 
Nevertheless, many plants continue to produce such 
organs in water cultures, even though the number of these 
organs or the individual length of each may be greatly 
reduced. In the soil so long as the plant does not wilt, 
it appears to be generally stimulated to most abundant 
root-hair production at a moisture content somewhat 
less than that which will afford the highest yield. 

It is commonly assumed that darkness is an immediate 
factor in the development of the root-hairs, but this as- 
sumption does not appear to be sustained by experiment. 
Relatively strong, diffused light with adequate water- 




Fig. 12. Root-tip of 
corn, diagrammatic. 



Water-Content of Plants 



49 



supply does not inhibit root-hair production in the ordinary 
crop plant ; but with strong light it is more difficult to 
maintain a high moisture content under laboratory con- 
ditions. 

32. The root-cap. — A longitudinal section of the tip 
p c 




Fig. 13. Root-tip of corn, high-power study of formative region : 
epidermis (e), cortex or periblem (c), plerome (p), and root-cap (r). 
[After Curtis.] 

of the root (Fig. 12) shows a rather complex but interest- 
ing structure. Protecting the growing tip, there is inva- 
riably found the well-known structure called the root-cap. 
The root-cap consists of a mass of cells to which falls the 
duty or office of a bumper organ. It is very effectively 
developed by divisions in the epidermal (protodermal) 
cells of the growing tip parallel to the surface. This mass 



50 Plant Physiology 

of cells is always resistant and compactly arranged within ; 
but as the cells are pushed outward, becoming old and 
sloughed-off by the continual addition of new cells, they 
undergo gelatinization and decay. This latter process, 
however, is important, for the gelatinization of these cells 
doubtless acts as a constant lubricant to make easier the 
course of the tip progressing through the soil. 

33. Structure of the root-tip. — In the root-tip proper 
there is a primordial meristem (Fig. 13) or region of rapid 
cell division. It is often called the formative region, and 
it is from this portion that the differentiation of tissues 
proceeds. In some cases the meristem is relatively ex- 
tensive, as in the pea and certain other legumes, while in 
the example given (corn), barley, sunflower, and others, 
it is limited to very few cells, — differentiated tissues 
extending practically to the morphological apex. 

The central portion of the root-tip is occupied by the 
plerome or central cylinder, a columnar mass of cells, 
most of which elongate considerably at a short distance 
back of the tip. In longitudinal section they appear as a 
rectangular type of cell in all that portion of the root 
unoccupied by root-hairs, but many of these cells are later 
transformed into the primary woody bundles. The cen- 
tral cylinder is surrounded by a cortical portion termed 
the periblem, made up at first of rather isodiametric par- 
enchymatous cells. The inner layer of this periblem is 
commonly differentiated by thicker walls to form a more 
or less definite sheath or endodermis. The external layer 
of the root-tip is the epidermis, also composed, in the 
immediate tip portion, of cells generally isodiametric. 
It is from these epidermal cells that tubular outgrowths 



Water-Content of Plants 



51 



develop as root-hairs. It is believed that factors operat- 
ing to increase rapidly the longitudinal extension of these 
cells will decrease the number and length of the root-hairs. 




Fig. 14. Cross-section of a rootlet: epidermis and bases of root-hairs 
(a), cortex (6), endodermis (c), and central cylinder (e) with xylem and 
phloem elements. 

It would seem that in some cases the retardation of growth 
by soil particles has a beneficial effect upon hair produc- 
tion. In a completely saturated air or in a high temper- 
ature — in other words when the conditions are such as to 
rapidly promote increase in length of the cortical cells of 



52 



Plant Physiology 



the root — there is a tendency to suppress hair pro- 
duction. 

34. Soil particles. — The extent of the root-system 
previously discussed enables the plant to come into con- 
tact with an enormous quantity of 
soil particles. Each particle of the 
soil, even air-dried soil, is invested 
with a film of water, and the amount 
jfr Jlm/Tal °^ wa * er which the soil may contain 

jjfjl %l fa is, to a considerable extent, depend- 
Igjwik }{& en ^ u P on the degree of fineness of 
W'\ ^v m. tnese particles. It is also dependent 
/ 1 %i upon the amount of organic matter, 

the so-called humus, as subsequently 

/ ! Tff"% j indicated. 

2* X With respect to the sizes of the 

particles, a soil may be designated 
as sand, silt, or clay. These three 
classes are subdivided into types 
or grades, depending upon the mi- 
nuter differences in size of the parti- 

Fig. 15. Seedling from a l 

sand culture, showing cles. The following table is generally 
adherence of grains. adopted by soil experts for the pur- 
pose of a mechanical classification : — 

Coarse sand &-A inch diam. 

Medium sand sJ-too inch diam. 

Fine sand iSinio inch diam. 

Very fine sand 255-*™ inch diam. 

Silt 555-5005 inch diam. 

Fine silt isWsoos inch diam. 

Clay mftnrwfero inch diam. 

All soils contain, in addition to larger particles, certain 




Water-Content of Plants 53 

proportions of these general constituents. In addition a 
fertile soil must contain a considerable amount of humus 
derived from the disintegration of plant and animal re- 
mains. 

35. Soil texture and water-holding capacity. — The 
water-holding capacity and the capacity for delivering 
water to the plant will depend, therefore, to a very con- 
siderable extent upon the mechanical constitution. In 
general, a fertile soil should consist of relatively fine par- 
ticles, since the water-holding capacity and the amount 
of food-materials will be, up to a certain point, a factor 
of the fineness of the particles. On the other hand, fine- 
ness of particles at or approaching the point of satura- 
tion of the soil may cause exclusion of air to such an 
extent that the finer the soil the less the amount of air 
present. In this connection, however, it should be re- 
membered that under ordinary circumstances the pore 
space of a clay soil may be 50 per cent of its volume, while 
that of a coarse sand uniform in texture may not exceed 
30 per cent. 

One cubic foot of an ordinary loam would cover a rela- 
tively enormous area if spread out as a single layer of 
particles. It is calculated that the contents of one cubic 
foot would cover about one acre. Some sort of mental 
picture of the absorbing system and its possibilities may 
be had by recalling, in conjunction with these indications 
respecting soil particles, the extent of the rooW^stem as 
clothed with root-hairs. The root-system of a mature 
sunflower may almost completely permeate a cubic yard 
of soil. 

It is necessary to make a comparison of some main soil 



54 Plant Physiology 

types with respect to the water-holding capacity in order 
properly to understand the relation of the plant to the 
soil. The water-holding capacity of the soil as ordinarily 
measured is the ratio of the weight of water, when the 
soil is at the point of saturation, to water-free soil, or dry 
weight of soil. 

The following table is an indication of what may be 
expected in this regard in certain general soil types : — 

Ordinary sand 20- 30 per cent 

Rich sandy loam .30- 40 per cent 

Rich clay loam 40- 60 per cent 

Very heavy clay 60-70 per cent 

Garden soil, rich in humus 70- 90 per cent 

Humus 100-300 per cent 

It is considered to be a general rule that a large number 
of cultivated plants find most favorable conditions for 
rapid growth when the soil contains from 40 to 50 per cent 
of its maximum water capacity. In the case of a sand, 
therefore, with a maximum capacity of 25 per cent the 
optimum for plant growth would be 10 to 12 J per cent. 
This will depend somewhat, however, upon the other con- 
ditions under which grown. 

Under the conditions encountered in the Laboratory and 
greenhouse, where in general the soil employed has been 
well stirred, or is constantly well aerated, the optimum 
moisture content may be much higher. In some cases 
this optimum may run as high as 70 to 75 per cent. Under 
field conditions it is apparent that the- amount of organic 
matter and the aeration of the soil are most important in 
determining the optimum. When organic matter is abun- 
dant in the soil, especially when the soil is compact, a 



Water-Content of Plants do 

tendency toward saturation apparently encourages a type 
of bacterial action which may promptly result in great 
harm to many agricultural plants. 

36. Exceptional plants. — The preceding statements 
relative to the optimum water-supply are to be understood 
as applying to a great maj ority of cultivated plants ; how- 
ever, as an example of an exceptional crop the cranberry 
may be cited. This plant grows to the greatest advantage 
in typical bog situations. As ordinarily cultivated, 
drainage is given this crop in such manner that the sur- 
face soil will not contain free water; yet under ordinary 
circumstances the soil approaches saturation on account 
of the low water-table, — at the same time bog conditions 
are such as to retard oxidation. The cranberry and many 
other bog plants are therefore adjusted to the peculiar 
conditions of their habitat. These bog conditions are, in 
fact, extremely interesting, but it is unnecessary to go into 
this subject further at this point. 

37. Unavailable water. — The plant is unable to with- 
draw all of the film moisture in contact with the soil par- 
ticles. If at any time the plant is unable to obtain from 
the soil the water it requires, wilting will ensue. The 
water then remaining in the soil is unavailable or non- 
physiological. When this point is reached, the soil is dry to 
the touch, yet an appreciable percentage of water remains. 
For any plant the film water unavailable in a variety of 
soils is proportional to the water-holding capacity of these 
soils ; that is, the greater the water-holding capacity the 
greater the pull against the plant when the content is low. 

Under ordinary agricultural conditions with loamy 
soils, there will be from 5 to 12 per cent of water unavail- 



56 



Plant Physiology 



able for most cultivated plants. This may be reduced to 
less than 1 per cent in coarse sand, and may rise to more 
than 50 per cent in typical New York muck. The tables 
which follow are after Heinrich ! ; the first shows the rela- 
tion existing between water capacity, hygroscopic water, 
and unavailable water ; the latter table gives a suggestion 
as to the conduct of different crops on two types of soils : — 



Watbb- 
content at 
Saturation 


Hygro- 
scopic 
Water- 
content 


Per cent 


Per cent 


26.5 


0.42 


43.9 


1.68 


41.4 


.97 


13.3 


2.40 


38.3 


3.65 


271.0 


20.60 



Water 

UNAVAIL- 
ABLE TO 

Plants 



Coarse sand 

Medium fertile garden soil 
Infertile sandy muck 

Sandy loam 

Very fertile calcareous soil 
Peat soil 



Per cent 
1.5 
4.6 
6.2 

7.8 

9.8 

49.7 



Plant 


Moisture Content at which Plants 
begin to Wilt 




( in ( lalcareous Soil 


On Peaty Soil 


Oats 

Barley 

Rye 

Red clover 

Potatoes 


Per cent 
8.4 
9.98 
9.55 
J 0.2s 
5.07 


Per cent 
32.3 
33.3 
32.8 

34.3 
41.4 



1 Cited by Cameron and Gallagher, Bureau of Soils, U. S. Dept. Agl. 
Bui. , 50 : pp. 57-58. 



Water-Content of Plants 



57 



It will be noticed that so soon as the amount of water 
in ordinary soils becomes about three times the hygro- 
scopic content it begins to assume physiological impor- 
tance. A soil which contains merely hygroscopic moisture 
is " air-dry " ; and if this amount only, or any amount 
less than " available," were present, the soil would actually 
withdraw water from the plant, thus inducing drying-out 
independent of transpiration. 

The following table, compiled from Hedgecock, includes 
certain agricultural plants as well as species inhabiting 
marshy and xerophytic conditions : — 

Plants grown in Loam, under Similar Greenhouse Conditions 



Plant 


Unavailable 
Water 


Coleus (Coleus blumei Benth.) 

Morning glory (Ipomcea purpurea Roth.) .... 

Cabbage (Brassica oleracea L.) 

Corn (Zea mays L.) 


3.0 
4.1 
5.8 
5.9 
5.9 


Wild rye (Elymus canadensis L.) 

Oats (Avena sativa L.) 

Asparagus (Asparagus officinalis L.) 

Lettuce (Lactuca sativa L.) 

Cucumber (Cucumis sativus L.) 

Arrowhead (Sagittaria latifolia Wild) 

Pondweed (Potamogeton americanus C. and S.) . 


5.9 
6.2 
7.0 

8.5 
10.8 
15.6 

24.8 



It will be noted that plants normally inhabiting water or 
swamp-land wilt first, and next to these are cucumber and 
lettuce, both with high-water requirement and relatively 
little structural protection against water-loss. 



58 Plant Physiology 

38. Leaves poorly fitted for water absorption. — In 
general leaves are of little practical value in the absorption 
of water. On a hot day a wilted plant recovers after a 
shower, not because it absorbs water rapidly through the 
leaf parts, but because (1) the atmospheric conditions then 
generally reduce the amount of transpiration, and (2) the 
roots are able promptly to get the water needed. Never- 
theless, partially wilted lettuce or peach leaves will be 
revived if the blades are dipped into water, even though 
the cut ends of the petioles are exposed to the air. A 
flax plant or a cabbage leaf would show no perceptible 
effect from the immersion for a long time. 

On the other hand some plants are capacitated for the 
absorption of water through leaves or pitcher-like vegeta- 
tive organs. Among these are certain of the Bromeli- 
aceae (a family to which the pineapple belongs) possessing 
leaves the bases of which sheathe the stem so closely as 
to form reservoirs for precipitation water. In this 
family, moreover, the absorption of water is more abun- 
dant by means of certain cells in the peculiar shield- 
shaped scales, and TiUandsia usneoides, the Florida moss, 
is an extreme form in this respect. It is an epiphyte, 
consisting of thread-like stems and narrow leaves, very 
common on trees in the far South. This plant is provided 
with much the same type of water-absorbing hairs which 
give the entire surface a glistening appearance. The 
aerial roots of orchids and some other tropical plants are 
provided with velamen, a chambered epidermal tissue 
which may absorb water like a sponge. 



Water-Content of Plants 



59 



LABORATORY WORK 

Water-content. — Determine separately the water-content of 
any convenient twigs and fruits, or of fruits and seeds ; — 
apples and apple twigs, squash and squash seed are generally 
available. If crude scales only are accessible, use from 50 to 
100 grams of material ; while if very delicate scales are em- 
ployed, 10 grams are sufficient. The process may be as follows : 




Fig. 16. A dry-heat sterilizer, serviceable also in determining water- 
content. 



(1) weigh and record weight of vessels to be employed, prefer- 
ably small glass, aluminium, or tin dishes ; (2) mince or break 
up the material finely, using a quantity more or less than that 
which is desired, into the proper vessel, weigh immediately, and 
record ; (3) dry at from 100 to 110° C. to constant weight (if a con- 
stant temperature dry-oven is at hand, leave in this until the 



60 Plant Physiology 

next period ; otherwise dry in any oven used for dry steriliza- 
tion during two successive laboratory periods), weigh, and 
record. If not weighed immediately, leave the material in a 
desiccator over fresh (dry) CaCl2 until determined. Calculate 
percentage of water. 

Determine the water-holding capacity of any rich garden 
loam, sand, and powdered quartz which may be available for 
later experiments. To completely saturate the material a sim- 
ple method is to fill a small pot or wire basket with the moist 
material, tamp down lightly, dip into (or pour on) water care- 
fully until slightly more than saturated ; then, as soon as the 
drip has ceased, dump approximately the quantity desired into 
the weighed vessel, and proceed as before. In calculating the 
water-content of soils it is to be remembered that at present 
the method more commonly employed is to determine the ratio 
or percentage of water, with respect to dry weight of soil, a 

calculation easily made as follows: r= — — f , in which w — 

w — w 

weight of saturated soil and vessel ; w , of dry soil and vessel ; 
w'\ of vessel ; r being the per cent of water. 

Root-systems. — The distribution of roots in the soil may 
be studied by careful excavation and washing out (cf. Ten 
Eyck, Kansas Agl. Exp. Sta. Bui. 127). A far better concep- 
tion of the fine root-system (under fairly natural conditions only 
when care is taken in the manipulation) may be obtained for 
demonstration, according to the method of Rotmistrov, by 
growing plants in flat zinc, or zinc-lined, boxes which for class 
purposes are large enough if 1 inch across, about 15 to 18 
inches wide, and IS inches deep. The box should contain sur- 
face-soil and sub-soil well compacted and arranged as in 
some local soil type. During the period of growth the boxes 
should be immersed to the surface in deep boxes of sand or soil. 
Growth should be permitted to proceed for a month or more. 
When the boxes are open, a side is removed, a fine mesh wire 
screen (about \ inch) is laid on the soil, the whole being inverted ; 
the soil is left upon the screen, and upon dipping this gradually 
into water an almost perfect root-system may be obtained. 



Water-Content of Plants 



61 



Root-hairs. — Germinate seeds of radish and barley in a 
germinator x (or upon sawdust or moss) and in moist soil. 





Fig. 17. 



Germinator for dry room : aerated chamber (A) ; saucer (B) ; 
and flower-pot (C). 



Compare under the microscope the length and appearance of the 
root-hairs, — the plants from soil being carefully washed and 
small rootlets mounted for observation. 

1 Germinators of a variety of types are employed in various physio- 
logical or seed-testing laboratories. In general, any device which secures 
constant moisture for the germinating seed is satisfactory, but in no case 
should any materials be employed which encourage the growth of molds 
or bacteria. Commonly it is sufficient to distribute the seed so that they 
shall not be in close contact one with another upon the surface of moist 
sphagnum moss, sawdust, or other moisture-holding materials of this 
nature into which the roots will readily penetrate. 

The type of vessel employed is of little consequence so long as there is 



62 Plant Physiology 

Root-cap. — Make a preparation of the whole root-tip of 
barley and note with the low-power of the microscope the 
general extent of the root-cap, the histology of which may only 
be studied by adequate sections. 

Root- lip. — Make longitudinal sections of the root-tip of corn 
or sunflower and compare with those of pea or vetch, distinguish- 
ing and describing primordial meristem, central cylinder, ple- 
rome, epidermis, and root-cap. Hand sections are sufficient, 
if properly made, but prepared slides are also desirable. 

Unavailable water. — Use young plants of cucumber, lettuce, 
and barley or wheat grown under similar conditions in small 
pots of rich or heavy loam and pure sand or quartz [preferably 
those soils the water-contenl of which were determined earlier]. 
The pots should contain no moss or such material at the 
bottom. When wanted for the experiment, the plants should 
be barely supplied with adequate water, so that during the 
laboratory period incipient wilting may begin. At the moment 
of wilting of each plant weigh out a portion of the soil of the 
pot and record the weight. These samples are retained and 
subsequently dried to determine the content of unavailable 
water in the different types. 

Water absorbed by leans. — Secure leaves of lettuce, peach, 

good aeration. Seed grown for physiological purposes should not be 
turned over or shaken during germination, as curvature of the roots is 
likely to result. In testing seeds for vitality, where numerous varieties 
or sorts are employed, special boxes with ruled spaces, each space de- 
signed for a given number of seed, are important. Information regard- 
ing this matter may be obtained from any bulletin on seed-testing. 

In the laboratory it is often desirable to obtain root-hairs in full 
development; in such cases flower-pots or special porous germinators 
(covered saucers) may be employed. With small seed, such as lettuce, 
mustard, etc., the procedure may be as follows: moisten the germinator 
or pot and sling a few seed against the inner surfaces. They will adhere 
to the surface at convenient distances apart, and then if provided with 
sufficient moisture, absorbed through the walls of the vessel, a rapid and 
vigorous germination will result, and the development of root-hairs will 
be shown in a simple and striking manner. 



Water-Content of Plants 63 

bean, cabbage, cudweed, or other plants available. Some of 
these should be leaves readily wetted, and others provided with 
a bloom, or with hairs effectively preventing wetting. Permit 
the leaves to wilt slightly, then weigh each kind accurately upon 
a delicate balance. Next place the leaves in water in a dark 
chamber, immersing all parts except the petioles (or, previous 
to weighing, seal the petioles carefully with wax). After from 
6 to 24 hours note any change in the rigidity in the leaves ; 
also remove all moisture from the surface with filter paper, 
weigh carefully, as before, and compare the weights of each 
kind. 

References 

Dandeno, J. B. Effects of Water and Aqueous Solutions of 

Some Inorganic Substances on Foliage Leaves. Trans. 

Canad. Inst. 7 : 238-350, 1901. 
Freidenfelt, T. Studien iiber die Wurzeln krautiger Pflanzen. 

Flora 91: 115-208, 1902. 
Hedgecock, G. G. The Relation of the Water-content of the 

Soil to Certain Plants. Botan. Survey of Nebr. 6:79 pp., 

1902. 
Hilgard, E. W. Soils. Pp. 188-310, 1905. 
Lyon, T. L., and Fippin, E. O. Soils. Pp. 136-165. 
Rotmistrov, V. Root-systems of Cultivated Plants of One 

Year's Growth. 57 pp., 22 figs. (Issued by the Experi- 
ment Station, Odessa, Russia.) 
Snow, L. M. The Development of Root-hairs. Bot. Gaz. 40 : 

12-48, pi. 1, 1905. 
Spalding, V. M. The Biological Relations of Certain Desert 
• Shrubs. I. Bot. Gaz. 38 : 122-160, 7 figs., 1904 ; II. Ibid. 

41 : 262-282, 1906. 
Ten Eyck, A. M. The Roots of Plants. Kansas Agl. Exp. Sta. 

Bui. 127 : 199-252, 26 pis., 1904. 

Texts. Goodale, Jost, Pfeffer, Sachs, Sorauer, Stevens. 



CHAPTER IV 

CONDITIONS AND PRINCIPLES OF AB- 
SORPTION 

The physical and chemical factors governing the ab- 
sorption of water and of solutions have long been the ob- 
ject of careful study. Many phases of such work have 
been developed in connection with plant physiology. The 
main facts, however, belong, in large part, to physics and 
physical chemistry ; yet an appreciation of the essential 
principles is necessary to an adequate understanding of 
the mechanism and work of absorbing organs. 

39. Imbibition phenomena. — Organic bodies of the 
most varied nature are able to take up water. Commonly, 
where this water is held within the body by capillarity or 
surface tension, and where there is produced also more or 
less swelling, the combined phenomenon is recognized as 
imbibition in a physiological sense. The hardest wood 
may absorb water by imbibition, and the force exerted in 
swelling or warping is capable of lifting or sustaining 
heavy weights; — sometimes made use of in splitting solid 
rock or stone. Dry seed-coats generally exhibit a high 
degree of imbibition, although these may be infiltrated 
with substances preventing the absorption of water, and 
in that way germination may be delayed. 

In the living plant imbibition phenomena are of im- 
64 



Conditions and Principles of Absorption 65 

portance, and particularly important in the activities of 
the nonliving cell-walls. The maintenance of the water- 
current or water-content of the plant is conditioned by 
imbibition, but under the conditions of growth the inflow 
of water into the cells of the root surfaces is effected by the 
force of osmosis, a mental picture of which is essential to 
an understanding of absorption phenomena. 

40. Osmosis and diffusion. — Osmosis and diffusion 
as generally understood determine the inflow of water to 
the root-hair, as well as that of the nutrients or other sub- 
stances (solutes) in solution. These forces are likewise 
important in the interrelations existing between cells. 

The fact of diffusion is readily observed. If a crystal of 
copper sulfate is placed in a tumbler of water, the salt goes 
into solution, and in time the colored solution diffuses 
itself equally throughout the water. This diffusion is 
wholly independent of any convection currents due to 
changes of temperature, and it is true for all such soluble 
substances as sugar, common salt, and the like. 

The movement of the particles of the dissolved sub- 
stances from the region of greater concentration to that 
of less implies a force, or pressure, which may be termed 
osmosis, or diffusion tension. 

41. The demonstration of osmotic pressure. — The 
osmotic action of a solute, in water as a solvent, may be 
conveniently demonstrated qualitatively by a simple 
experiment in which it is made evident as hydrostatic 
pressure. A factor not yet mentioned which is involved 
and which is absolutely essential in this demonstration is 
a semipermeable membrane, an imbibition membrane 
which, in this case, permits water to pass through readily, 



66 



Plant Physiology 



but is impermeable, or very slowly permeable, to the sub- 
stance in solution. Semipermeable membranes are of 
the most diverse sorts. A piece of 
pig's bladder, a commercial article 
readily obtainable, is a very satis- 
factory membrane. This, after be- 
ing soaked in water, is tied tightly 
over the bell end of a thistle-tube 
(Fig. 18) with waxed thread. The 
solution to be tested, preferably a 
known strength of some solute, say 
20 per cent sugar, is carefully 
poured into the stem of the thistle- 
tube (easily accomplished with a 
guide-wire, or without when the 
tube is clean and wet) until the 
bell is filled with the sirup. The 
tube is then lowered into a vessel 

a G „d 18 -th°lt° b n e "£ ° f ™ ter UlltiI '^h fluids, Which 

demonstration osmo- should have come to room tem- 
scopes ' perature, are at the same level, when 

the tube is clamped to a support. It is well to add about 
1 per cent of formalin to both liquids as a preservative. 
If the experimental conditions are properly carried out 
the liquid in the thistle-tube will rise perceptibly in a few 
hours, and an experiment to be continued a day or more 
will require at the outset an extension of tubing. There 
is, therefore, a major flow of water through the membrane 
to the strong solution; thus there is manifest a pressure 
which, if it could be completely measured by this column 
of water in the apparatus, would be the osmotic pressure 




Conditions and Principles of Absorption 67 

of the 20 per cent sugar solution. If there were a less 
concentrated sugar solution in the outer vessel, or any 
solution of salts containing fewer solvent particles per 
volume, the major flow of water would still be inward, and 
the extent of this pressure would be in direct proportion to 
the difference in number of solvent particles. 

Special diffusion shells (Fig. 18) are also prepared for 
demonstration experiments, and these likewise give quick 
results. In this connection it might be mentioned that 
accurate measurement of osmotic pressures involves 
special apparatus in which the natural membrane is re- 
placed by an artificial precipitation membrane deposited 
in the. interstices of a porous cup, the pressure being indi- 
cated by a mercuric manometer. The original of this 
apparatus, described by Pfeffer in 1877, has been greatly 
improved in recent years by Morse. 

42. An explanation of osmotic pressure. — In the 
thistle-tube demonstration experiment it was suggested 
that the molecules of sugar or other solute tend to diffuse, 
to distribute themselves equally, but the semipermeable 
membrane retards and almost prevents this outward 
diffusion. The inflow of water is conceived to be in direct 
response to this force, analogous to a pressure, and the 
water would continue to flow inward until equilibrium 
between the pressures were established. This commonly 
accepted view of osmosis regards the solute as obeying 
the laws of gases. At a constant temperature, therefore, 
the osmotic pressure varies with the densitj^ or concentra- 
tion; that is, with the number of particles of the solute. 
The molecular weight of cane-sugar (a nonelectrolyte) 
in grams, 342, dissolved in water to 1000 cc. (called a gram- 



68 Plant Physiology 

molecular solution, or M solution), would give, according 
to this, an osmotic pressure of 22.4 atmospheres. 1 Ordi- 
narily the osmotic pressure of an epidermal cell of a leaf 
or of a meristem cell is somewhat more than 4 atmos- 
pheres, or about .20 gram-molecular (7 per cent) of sugar 
as determined by the method subsequently discussed. 

The plant cell behaves very much as the simple os- 
mometer above described. The root-hair, for example, is 
a case in which the cell-sap is the strong solution, the limit- 
ing layer or edge of cytoplasm is the plasmatic membrane 
(the cell-wall in this instance furnishing support and pro- 
tection), and the soil water is the weak solution. The 
flow of water is into the cell ; in fact, under such circum- 

1 A gram-molecular solution of such substances as potassium nitrate 
or common salt (electrolytes) yields a pressure higher than 22.4 atmos- 
pheres. This is explainable on the ground that the number of particles 
in solution is increased by the partial or complete dissociation of the 
molecules of such substances into their ions. Thus KXO3, when partially 
dissociated, yields in addition to KNO.s the ions K + and NO3 - . Gram- 
molecular solutions of this salt show a pressure of about 35 atmospheres. 
For this reason cane-sugar and potassium nitrate are not osmotically 
equal; that is, isosmotic at the same molecular strength. Before the 
theory of dissociation was developed De Vries determined that organic 
substances such as sugar have about two thirds the osmotic value of 
monovalent salts ; that is to say, that the ratio of their coefficients is as 
2 to 3. This ratio and those developed by De Vries for dibasic or other 
compounds are fairly satisfactory as indications of the isosmotic relations 
at the strengths corresponding to the plasmolysis of higher plants. 
Nevertheless, since the per cent of dissociation varies considerably 
among the salts of the monobasic, dibasic, or other groups, it is essential 
in any comparative quantitative work to know accurately the per cent of 
dissociation of any electrolyte employed. This may be obtained from 
physical chemical tables. The discussion of the relation between elec- 
trolytes and nonelectrolytes and the formula for comparing the latter 
with the former is developed in the work by Livingston, cited in the 
literature. 



Conditions and Principles of Absorption 69 



stances, so long as an equilibrium has not been established, 
and water is available, there is absorption of water, and 
there is manifest always, with adequate water-supply 
within the cell, an hydrostatic pressure known as turgor. 
This turgor, existing throughout the plant, is, as already 
indicated, the chief cause of the rigidity of leaves and suc- 
culent shoots. Turgor is then the expression of the os- 
motic pressure of the cell. This turgor may be measured 
by simple experiments conducted as described below. 

43. Plasmolysis and wilting. — If the root-hair or any 
equivalent cell is placed in a solution stronger than the 
cell-sap, the major current of water will be outward, so 
that water will be withdrawn from the protoplasm, the 
latter contracting from the cell-wall. This state of con- 
traction is termed plasmolysis. Plasmolysis throughout a 




Lettuce plants in solutions : A, tap water ; B, 2.9 per cent 
sodium chlorid. 



70 Plant Physiology 

tissue or organ results in flaccidity or wilting. In Figure 
19 are shown two lettuce plants transferred from soil. 
The roots of plant A were put into water, and those of B 
into 2.9 per cent sodium chloric! (approximately .5 gram- 
molecular). The latter has caused a prompt loss of 
water by the plant, so that wilting has resulted. 

If slices of beet or potato are placed in solutions similar 
to those just mentioned, tumescence on the one hand and 







Fig. 20. Successive stages in plasmolysis: epidermis of Tradescantia 

(a) and cells of Spirogyra (6). 

flaccidity on the other will result in the same manner. 
The phenomenon of plasmolysis in the cell is sufficiently 
important to be carefully studied. It is often more readily 
observed in a cell with colored contents, or numerous 
chloroplasts, so that the cells of filamentous alga?, the 
colored epidermis of certain begonias, or of Tradescantia 
zebrina, also the stamen hairs of Tradescantia and Ana- 
gallis, are convenient materials for demonstration. Figure 
20 shows successive stages in the plasmolysis of the epi- 
dermal cell of Tradescantia zebrina and of Spirogyra. 
The concentration which will just cause the least trace of 



Conditions and Principles of Absorption 71 

plasmolysis is taken as the measure of the turgor of the 
cell. 

Plasmolysis may be regarded as a general phenomenon, 
yet it should be observed that the cytoplasm may undergo 
contraction, wholly independent of osmotic relations, under 
the influence of certain stimuli. Greeley l ascertained 
that low temperature may produce this effect, and under 
certain circumstances high temperature may also cause 
contraction. It has long been known that injurious sub- 
stances may produce plasmolysis coincident with injury, 
a fact to which also Osterhout 2 has recently called atten- 
tion. 

44. Variation in turgor. — Considering plants as a 
whole, thus including fungi, algae, and the higher plants, 
there is great variability in turgor. The fungi show the 
greatest range and are therefore adapted to thrive in the 
most diverse situations with respect to concentration. 
Fresh-water algae and all the higher plants show, on the 
whole, a less extensive range; yet even in the same plant 
the cells of different parts, or tissues, may show no signifi- 
cant variation. Commonly, as already suggested, the tur- 
gor in many epidermal and parenchyma cells is equiva- 
lent to about 7 per cent of sugar (4.5 atmospheres, but 
often the range may be from 4 to 8 atmospheres). This 
fairly close range in the higher plants is perhaps to be 
anticipated, since it is not conceivable that the strength 
of the soil solution, under the complex physical and chemi- 
cal conditions, would show unusual extremes. It may be 
readily shown by experiment, however, that in a strong 

1 Greeley, A. W., Am. Journ. Physiol., 6: 112-128, 1901. 

2 Osterhout, W. J. V., On Plasmolysis. Bot. Gaz., 46 : 53-55, 1908. 



72 Plant Physiology 

solution plants develop a somewhat higher turgor than 
when grown in an extremely weak solution. 

According to the work of De Vries many plant cells are 
just plasmotyzed at a concentration of about 1.2 to 1.4 
per cent KN0 3 (.12 to .14 gram-molecular solution), 
which is equivalent to about 5 atmospheres. Active cells 
of the cambium may require a concentration of from 
.4 to .5 gram-molecular solution ; and in an investigation 
of medullary ray cells in a common willow (Salix fragilis) 
Kny finds that the different types of these cells are plas- 
molyzed at concentrations varying from .10 to .8 M. 
Moreover, turgor varies with age, nutrition, and with 
environmental factors, such as heat and light. 

In general, observations upon living cells render it 
perfectly obvious that turgor is usually an essential attri- 
bute of active cells. There is, unquestionably, an impor- 
tant interrelation between turgor and growth; therefore, 
conditions affecting turgescence affect simultaneously 
all growth processes. 

45. Substances active in producing turgor. — Xo one 
compound or group of compounds is responsible for turgor. 
It may be due to dissolved substances both organic and 
inorganic, and while, in some cases, it does not change 
materially during the growth or other activities of the 
cell, yet the composition of the sap may undergo a rela- 
tively great change. According to Pfeffer, the turgescence 
of cells in the root of the sugar-beet is produced largely 
by cane-sugar, while in the sunflower more than 40 per 
cent may be represented by potassium nitrate. The beet 
contains relatively little sugar when young, and the sun- 
flower little nitrate. Various other inorganic salts, glu- 



Conditions and Principles of Absorption 73 

cose, organic acids, and many other compounds are also 
important in the osmotic strength of cells. The high 
turgor of certain mold-fungi growing upon concentrated 
solutions has been determined to be due to organic sub- 
stances, which may be readily produced within the cell. 

46. Osmosis and the absorption of nutrient salts. — 
The water requirement is not the only one with which 
osmosis is concerned, for the principles of osmosis and 
diffusion govern also the absorption of nutrient salts; 
likewise, of course, the absorption of any other substances 
present in the soil solution. Moreover, the plasmatic 
membrane is to a degree permeable to all the nutrients, 
and to many other substances as well. 

Each substance in the soil solution has its specific tend- 
ency to diffuse, and it therefore tends to come to equilib- 
rium with the tension of the same substance in the cell. 
The cells which are active in absorption have in turn a 
relation to those adjacent to them, and this relation, 
emphasized or otherwise modified by cells especially ca- 
pacitated for conduction, extends to all parts of the com- 
plex organism. The root-hair, then, in so far as it is 
permeable, absorbs each substance or solute particle inde- 
pendently, and in accordance with a certain attraction for, 
or use of, that substance in some way, as in the deposition 
in an insoluble form — it may be in the building up of pro- 
toplasm, or in the accumulation of complex food-materials. 

One of the most remarkable facts respecting the osmotic 
relation of the plant to the soil solution is that there is so 
little exosmosis, or outward diffusion of substances from 
the plant, — substances present in the plant but not in 
the soil. Again, it is difficult to understand the absorp- 



74 Plant Physiology 

tion, transport, and final accumulation (often without 
change) of certain substances in special organs. There 
are a number of factors affecting such relations, but much 
is yet unexplainable on a physical basis. 

47. Protoplasmic permeability. — It is an obvious fact 
that the plasma membrane is permeable to certain solutes, 
else no growth could result, It is as clearly apparent that 
this membrane is impenetrable to certain other solutes, 
and this implies selective absorption. The fact of imper- 
meability becomes evident from a simple observation upon 
colored cell-sap, and especially so upon contemplation of 
the phenomenon of plasmolysis in cells containing colored 
sap. The colored cell-sap of a red beet or of a cell from 
a stamen-hair of Tradescantia does not diffuse into the 
surrounding water so long as the cell is uninjured. More- 
over, when such cells are plasmolyzed, there is, with con- 
tinued health, no noticeable exosmosis of the colored 
material. From dead cells there is prompt diffusion of 
the colored sap. In this connection it is also to be re- 
membered that in many cases red or blue color in plant 
cells is merely an indication of acid or basic substances, 
and this color may be changed in the living cell if it is per- 
meable respectively to basic or acid compounds. 

Pfeffer has clearly demonstrated important facts regard- 
ing permeability through his experiments upon the pene- 
tration of dye stuffs. Methylene blue at a strength of 
1 part to 100,000 of water yields a solution which is not 
visibly blue unless observed in a layer several centimeters 
thick. It would not therefore give an evident coloration 
in a plant cell. It is found, however, that upon being 
placed in such a solution certain root-hairs, Spirogyra, 



Conditions and Principles of Absorption 75 

and other cells are quickly colored blue. It is evident that 
there has been penetration, and further that there has 
been accumulation of the dye. In some cases this accu- 
mulation is particularly noticeable, due to the formation 
of a granular precipitate, as in Spirogyra. These facts 
give some faint idea of the complexity of the problems of 
cell absorption. 

In accordance with the foregoing statements it is pos- 
sible to assume that when a mixed solution is presented 
to a root-hair, certain substances, independent of their 
concentration in the environment, may be absorbed, while 
others, whether dilute or concentrated, will fail to enter. 
Upon this ground the relatively abundant occurrence of 
iodine in seaweeds may be explained. In seawater iodine 
is present at very great dilution, about .000001 ; yet it 
is accumulated in marine alga? to such an extent that it 
has yielded (and still yields upon the coasts of Japan) a 
commercial source of this material. Similar and striking 
examples may be found from a study of the ash content 
of an}^ plant; thus the content of potash, iron, phos- 
phoric acid, etc., may be greater than the ratio of these 
substances in the soil solution, whereas other substances 
may be absorbed in relatively less quantity. The ash 
content of plants, however, is discussed at greater length 
later. 

It is scarcely practicable to consider here some of the 
factors which have been found to affect permeability and 
selective absorption. It is necessary to observe, however, 
that Overton has developed an interesting theory of 
absorption based upon certain facts. One of these facts 
is that substances may be assembled into diverse groups 



76 Plant Physiology 

with respect to permeability, and there has seemed to be 
some relation between the capacity to penetrate and the 
solubility of the solute in cholesterin or other similar com- 
pound. This has naturally led to the assumption that 
some such substance constitutes an important part of the 
plasma membrane. Nevertheless, there are many ex- 
ceptional cases, and different plants frequently exhibit 
marked specific peculiarities. 

The differences in penetration referred to are charac- 
teristic also of toxic or injurious compounds as well as of 
nutrient or beneficial substances. This fact is frequently 
of service in explaining the relative toxicity of different 
reagents. Nowhere is this shown more clearly than in the 
experiments of Brown, from which it is evident that the 
seeds of barley may be placed for a considerable time in a 
relatively strong solution of sulfuric acid without injury, 
whereas mercuric bichloride rapidly effects an entrance 
and kills the cells. Further details of this experiment 
are cited later (section 263). The illuminating experi- 
ments of Kahlenberg on osmosis demonstrate clearly that 
the nature of the semipermeable membrane is a matter 
of great importance in osmotic phenomena. Further- 
more, it has been shown that external conditions, includ- 
ing those of temperature, light, and nutrition, affect 
permeability and selective absorption to a high degree. 
The plasma membrane should be regarded as made up 
in part of a variable and complex colloidal solution. 

48. The role of diffusion and osmotic pressure. — From 
what has been brought forward respecting osmosis and 
diffusion it can be said that these forces are conspicuous 
in the work of the cell. The concentration of the cell- 



Conditions and Principles of Absorption 77 

sap above that of the soil solution, or other liquid environ- 
ment, conditions a turgor, an expression of osmotic force. 
This turgor is coexistent with growth. It likewise confers 
upon cells or organs a substantial rigidity. The concen- 
tration of osmotically active substances manifest through 
the absorbing surfaces represents a constant pull upon the 
environment for water, so that root-hairs are able to ab- 
stract water from surfaces or solutions which do not repre- 
sent a greater pull. The plasmatic membrane is extremely 
complex with regard to permeability, and it may exhibit 
marked powers of selective absorption. In simple (few- 
celled) plants osmosis and diffusion may be all-sufficient 
in what is practically the movement of solutions, but in 
higher plants there are, in addition to these important 
forces, also other factors affecting mass movement along 
the special conducting paths of the fibrovascular system, 
as noted later. 

49. Sap or root pressure. — The absorptive capacity 
of the root, conditioned by its osmotic relations, may give 
rise to a pressure, termed root pressure or sap pressure, 
which may be manifest within the plant whenever the 
greater rapidity of transpiration does not create a nega- 
tive tension. 

Bleeding phenomena are evidences of this pressure. 
During the spring, in particular, the maple, birch, grape, 
potato, black nightshade, nettle, and a variety of other 
woody and herbaceous plants bleed profusely. In some 
cases bleeding is checked by drying-out, by the deposition 
of solid or glutinous matter, and by growth processes (ty- 
loses) filling up the vessels from adjacent cells. In other 
instances corky layers may be formed sooner or later. 



78 



Plant Physiology 



The amount of the exudation may vary from a few 
drops to several liters per day. Large quantities have 
been reported for a few plants, especially tropical or sub- 
tropical forms; thus Humboldt reports for the American 
aloe 7.5 liters per day, or about 1000 liters during the 
entire period; while if the observations of Semler are 
taken, Caryota urens may produce 50 liters per day, the 
maximum amount observed. Among agricultural plants 
employed in demonstration work, the potato and tomato 
are good for short observations, and the grape vine — 
less subject to decay — for more extended experiments. 
Eckerson finds that among common greenhouse species, 
Fuchsia speciosa and Begonia coccinea are especially favor- 
able for quantity. The pressure under which the exuda- 
tion is produced necessarily bears no relation to quantity 
of exudate. The following table, taken from the data of 
Eckerson, indicates what may be expected of satisfactory 
material in experimental studies : — 




Begonia coccinea (Begonia) .... 

Chrysanthemum frutescens (Marguerite) 

Fuchsia speciosa (Fuchsia) .... 

Helianthus annuus (Sunflower) . 

Lycopersicum esculentum (Dwarf Stone 
tomato) 

Pelargonium zonule (Horseshoe gera- 
nium) 



Conditions and Principles of Absorption 79 



A demonstration of the quantity of liquid pro- 
duced, and of the existence of root pressure, may 
be made by comparatively simple methods. The 
quantity is readily determined by cutting off the 
plant an inch or two above the surface of the 
ground and connecting the stump by rubber and 
glass tubing with a measuring glass protected 
against evaporation. For the proper demon- 
stration of pressure a suitable manometer is re- 
quired (Fig. 21). 

LABORATORY WORK 

Imbibition; swelling of wood. — Use small blocks of 
oak, basswood, and pine, practically cuboidal in form, 
preferably cut so that tangential, radial, and longi- 
tudinal axes are represented. Mark opposite points 
of each axis with a pencil and measure carefully with 
the calipers provided. Then soak the 
blocks in distilled water for from five 
to ten days, changing water each day ; 
after which, remeasure each axis and 
compute the percentage of change. 

Heat of imbibition. — Reduce 100 
grams of common starch to a uniform 
powder, dry in an 
oven at about 105° C, 
and at the same time, 
for a control experi- 
ment, dry 100 grams 
of quartz flour or 
graphite. Cool both 
powders to room tem- 
perature in a desic- 
cator, and pour each 
. J^ig. 21. Uanong's manometei. [After the 

into a Dewar flask or Bausch and Lomb Optical Company.] 




80 Plant Physiology 

small thermal bottle (a tumbler may be used when double- walled 
vessels are unavailable). Take the temperature of each powder, 
then add 100 cc. of water at the same temperature, stir promptly 
with a clean wooden stirring rod (the starch mixes with water 
less readily), observe the temperatures, compare, and discuss the 
results. 

Osmoscope. — Set up an osmoscope as indicated in section 
41, using a thistle- tube and membrane, or a diffusion shell. 
Different strengths of sugar solution, 20, 40, and 60 grams per 
100 cc. of water, may be used to note differences in rate of flow 
and total height of column, but no accurate quantitative results 
are to be expected. Describe the results obtained. 

Precipitation membrane. — Drop a crystal of copper sulfate 
into a bottle containing 5 per cent potassium ferrocyanide, and 
observe the formation of a semipermeable precipitation mem- 
brane of copper ferrocyanide, and the prompt rise of an irregu- 
lar column of solution inclosed by this, which grows and may 
attain considerable proportions in fifteen minutes. More neatly, 
the precipitation membrane may be studied by employing a 
more dilute solution of potassium ferrocyanide (2 per cent) in a 
dropper bottle into which is lowered cautiously to its position a 
dropper tube with capillary outlet, containing a single drop of 
strong copper sulfate. Note and describe the phenomena tak- 
ing place. 

Plasmolysis and wilting. — Prepare 250 cc. of .5 gram- 
molecular (M.) solutions of potassium nitrate and of sodium 
chlorid as stock solutions. From these solutions make dilu- 
tions in small vials, capacity about 25 cc, to contain the fol- 
lowing strengths of each of the above solutions, namely, .10, .20, 
.30, and .40 molecular (M.) ; also one -vial with distilled water 
as a control. In each of the dilutions place a seedling of some 
plant (root as nearly entire as possible) with delicate stems, or 
leaf stalks, such as lettuce, radish, or mustard. Observe the 
dilutions in which wilting occurs, and note the time required in 
the solutions in which it occurs. Compare the equivalent 
strengths of the two salts. The above experiment will illustrate 
the withdrawal of water by strong solutions and will suggest 



Conditions and Principles of Absorption 81 

the progressive plasmolysis and wilting of the cells of the plant 
through the root-system, but the osmotic strength of the cell- 
sap may be more accurately studied through the next two 
experiments. 

Osmotic pressure of cell-sap; observation upon tissues. — From 
the stock solutions used in the preceding experiment prepare in 
stender dishes or Syracuse watch glasses dilutions which shall 
contain the following strengths, .10, .12, .15,. 18, and .20 molec- 
ular (M.). Split the apical portions of several flower stalks 
of the dandelion (or other scape which has been found suitable) 
each into four approximately equal parts. These strips will 
curve outward, the epidermis being within or on the concave 
side. Dip strips momentarily into water in which spirals will 
be formed, then cut into distinct rings. Place one or two of 
the rings in each of the above solutions and also in distilled 
water. Follow and note the changes which occur. Further 
curling of the strips indicates absorption of water, that is, the 
solution is too weak ; no change in the curvature indicates a 
solution equal in osmotic strength to the cell-sap (isosmotic 
with the cell-sap) ; and elongation or reverse curvature indicates 
loss of water and plasmolysis. Intermediate dilutions may also 
be made, and the threshold of plasmolysis more accurately 
determined. 

Osmotic pressure of cell-sap; direct observation upon plas- 
molysis. — The osmotic pressure of the cell-sap may be deter- 
mined fairly accurately by direct observation upon the 
plasmolysis of the cell, employing as the plasmolytic agents 
substances which penetrate the cell only very slowly. The 
substances employed above, also other neutral salts, cane-sugar, 
etc., may be used. Cells with protoplasts the limits of which 
may be easily seen are best for preliminary study, especially 
algae, such as Spirogyra, Pithyophora, etc. All precautions as 
to the cleanliness of vessels, also purity of the reagents and 
distilled water, should be observed. 

From the stock solutions of the monovalent salts previously 
used prepare for a preliminary test a small quantity of a .2 M. 
solution. Mount in a drop of this solution one or two filaments 

G 



82 



Plant Physiology 



of the alga, observing under the microscope for ten minutes 
whether there is or is not some plasmolysis. Then, according 
to the result, prepare dilutions of 
less or greater concentration, and 
determine accurately the thresh- 
old of plasmolysis. For accurate 
work the hanging-drop culture 
may be employed. Determine 
the osmotic strength also in terms 
of cane-sugar. Peel off some of 
the lower epidermis (with colored 
cell-sap) of Cyclamen or Trades- 
cantia zebrina, or use leaf hairs of 
a cucurbit, and determine the os- 
motic strength of these cells. 

With cells of any plant just 
distinctly plasmolyzed determine 
if turgor may be restored by 
irrigation with tap or distilled 
water. 

Shrinkage. — With any of the 
above plant material mounted in 
water measure accurately with 
the ocular micrometer a cell easily 
located. Draw off the water and add successively stronger 
salt solutions until approaching the point of plasmolysis ; re- 
measure ; plasmolyze the cell, and again measure. Compare 
the results with respect to shrinkage. 

Protoplasmic permeability. — Into a solution of methylene 
blue, 1 part to 100,000 parts of water, place a seedling of radish 
or mustard with well-developed root-hairs; also filaments of 
Spirogyra and a sprig of Elodea. In two hours examine the 
root-hairs, the cells of Spirogyra, and the leaf cells of the Elodea 
for penetration of the dye, and discuss the results. 

Sap or root pressure. — Utilizing suitable plants in the open, 
or potted specimens, determine the amount of water exuded 
upon decapitation, and also the pressure of exudation in two 




Fig. 22. Simple method 
demonstrating exudation 
from a decapitated plant. 



Conditions and Principles of Absorption 83 

species of plants (see section 49). In determining the amount 
of exudation, conduct in each case the liquid into a graduated 
test-tube Avith foot, in which test-tube is placed a drop or two 
of oil to prevent evaporation. In the pressure determination 
employ a Ganong manometer, or one similar in principle im- 
provised from materials at hand. Observe frequently, calculate 
the pressures at the different intervals, and draw curve of 
results. z 



References 

Eckerson, S. H. Root Pressure and Exudation. Bot. Gaz. 45 : 

50-54, 1908. 
'Kahlenberg, L. On the Nature of the Process of Osmosis and 

Osmotic Pressure. Journ. Phys. Chem. 10 : 141-209, 1906. 
Livingston, B. E. The Role of Diffusion and Osmotic Pressure 

in Plants. Dec. Publ. Univ. Chicago. 8 : 149 pp., 1903. 
Nathansohn, A. Ueber die Regulation der Aufnahme anor- 

ganische Salze durch die Knollen von Dahlia. Jahrb. f. wiss. 

Bot. 39 : 607-644, 1904. 
Overton, E. Ueber die osmotischen Eigenschaften der Zelle. 

Festschrift Naturf. Gesellsch. Zurich. 1896. 
Pfeffer, W. Osmotische Untersuchungen. 236 pp., 1877. 
Kuhland, W. Beitrage zur Kenntniss der Permeabilitat der 

Plasmahaut, Jahrb. f. wiss. Bot, 46 : 1-54, 1909. 
Vries, H. de. Eine Methode zur Analyse der Turgorkraft. 

Jahrb. f. wiss. Bot. 14 : 427-601, 1884. 
Van't Hoff, J. H. Die Rolle des osmotischen Druckes in der 

Analogie zwischen Losungen und Gasen. Zeitsch. f. phys. 

Chemie. 1 : 481-508, 1887. 
Wachter, W. Ueber den Austritt von Zucker aus den Zellen 

der Speichorgane von Allium Cepa und Beta vulgaris. Jahrb, 

f. wiss. Bot. 41 : 165-220, 1905. 

Texts. Barnes, Ganong, Jost, Pfeffer. 



CHAPTER V 
TRANSPIRATION AND WATER MOVEMENT 

The water-content of a plant is no index of the amount 
which has been absorbed throughout its life by the root- 
system. It is a thoroughly familiar fact that water is 
commonly eliminated from the plant as water- vapor. 
This elimination, termed transpiration, is important and 
should receive special consideration. A very large pro- 
portion of the water absorbed by plants is transpired; 
that is, it passes into the atmosphere by diffusion through 
the leaves and other delicate parts. This loss of water 
maybe very simply demonstrated by placing a potted plant 
under a bell glass, taking the precaution to place a rubber 
cloth over the pot and over all possible evaporating sur- 
faces except the plant itself. In a short time a mistiness 
upon the glass will indicate roughly the loss of water. 

50. Observations upon transpiration. — The demon- 
stration of water-loss may be made in a variety of ways, 
best of all by loss of weight. Nevertheless, single leaves 
and abscised branches or organs may be employed in 
various potometers, by means of which there is measured 
the water absorbed, this latter corresponding in the end, 
of course, very closely to that which is given off. Interest- 
ing experiments may be readily set up with single leaves 
or shoots (Fig. 23). By another type of experiment 
84 



Transpiration and Water Movement 



85 



individuals growing in the 
field l may be made the 
objects of observation and 
comparative study. 

Experiments made with 
abscised branches may not 
be typical, for the shoots are 
in abnormal relations, lack- 
ing the usual organs of absorption, as 
well as the special soil conditions ; and 
since there is, further, a certain re- 
sponse to the injury received, the 
results of experiments made with plant 
parts do not, perhaps, represent the 
loss under natural conditions. These 
parts may be employed, nevertheless, 
for demonstration and for determining 
more or less accurately the relative 
rate of loss under different conditions. 

Transpiration may be most accu- 
rately determined by using potted 
plants, observing the precautions in- 
dicated with respect to evaporating 
surfaces, and weighing at successive 
intervals. Special recording balances 
have been constructed and used for 
this purpose, but ordinarily such de- 
vices are unnecessary to demonstrate 
principles and limiting conditions. 

1 Freeman, G. F., A Method for the Quan- 
titative Determination of Transpiration in 
Plants. Bot. Gaz., 46: 118-129, 1908. 




Fig. 23. Burette po- 
tometer ; shoot fitted 
with rubber tissue. 



86 



Plant Physiology 



On a large scale, a rapid loss of water from plants is 
familiar to all in the process of hay-making. The differ- 
ence in weight between green and dry hay is perfectly 
obvious. There may, of course, be a slight loss of water 
from the cut surfaces of the stems, but even should these 
be sealed by paraffin or wax, wilting and loss of water will 
proceed almost as rapidly as before. Practically all parts 




1 1 1 1 ■ m 1 i ' i i ;^ 




Fig. 24. Potometer with shoot-chamber (A), small-bore record-tube 
(B), water-reservoir (f), and stop-cock for refilling tube (Z>), sup- 
ported by base (E). [Adapted from Ganong.] 



of plants lose water to at least a slight extent. Apples or 
potatoes stored in a fairly dry situation during a consider- 
able period of time will show considerable loss, although 
the normal surfaces of such parts are so constructed that 
rapid drying-out is prevented. 

As soon as wilting takes place, sufficient practically 
to close the stomata, the rate of loss will drop, and thus 



Transpiration and Water Movement 



87 



the effect of closure of the stomata is made evident in the 
otherwise more or less normal curve of evaporation. 

51. Amount of transpiration. — According to Haber- 
landt, a corn plant may transpire during a single growing 
season 14 kg. of water, a hemp plant 27, and a sunflower 
66. 1 That is to say, a sunflower may transpire more than 




Fig. 25. Portions of epidermis stripped from a leaf of Cyclamen: upper 
epidermis to the left (no stomata), lower epidermis to the right 

(6 stomata). 

500 grams per day throughout its entire season, which would 
mean a very much greater amount during a day of maxi- 
mum loss. Estimated from the transpiration of a small 
plant, an apple tree of, say, 30 years old might lose 250 
pounds per day, possibly 36,000 pounds during a growing 
season. Therefore, one acre of 40 trees would represent a 

1 The indications are that these figures are far too low for conditions 
in the United States generally. 



88 Plant Physiology 

water elimination of about 600 tons. Land covered by 
grass or clover may lose during the growing season from 
500 to 750 tons of water, almost entirely through the sur- 
faces of the growing plant, 

52. The mechanism permitting transpiration. — The 
elimination of water from the surfaces of plants takes 
place because of the fact that the leaves or other surfaces 
are not wholly impermeable to water- vapor. In the case 
of delicate, especially young, leaves or shoots there may be 
some loss of water directly through the epidermis, which 
is then relatively uncutinized, or otherwise unprotected 
against water-loss. In many instances this amount is 



Fig. 26. Section of tomato leaf: epidermis (e), palisade tissue (p), paren- 
chyma (g), vascular bundles (i), and stomata (s). 

negligible, and just as the continuous cuticle commonly 
absorbs practically no water, so it does not permit of elim- 
ination. The epidermis, however, of one or of both sur- 
faces of the leaf and of other delicate parts may be pro- 
vided with numerous pores or stomata (Figs. 25 and 26) 
which are the most important means of communication 
between the internal tissues and the external air. 

The stomata open and close in response to complex 
internal conditions, and under certain circumstances 
external factors may perhaps play at least a secondary 



Transpiration and Water Movement 89 

role, as later developed. They usually open into a sub- 
stomatal cavity which, in turn, is in communication with 
the intercellular spaces, or aeriferous system. Since the 
leaves are the organs commonly active in transpiration, 
it is necessary to note the structure in a typical case. 

In Figure 26 there is shown a cross-section of the leaf of 
tomato. There is a single epidermal layer (e) on each 
surface, a single palisade layer (p), and the mesophyll or 
leaf parenchyma. Small veins, or fibrovascular bundles, 
in cross and longitudinal section are also shown. In the 
lower epidermis there are several stomata. Many leaves 
show a multiple palisade, and there is considerable diver- 
sity generally in the form and compactness of the tissues. 

Each cell of the leaf is directly or indirectly in contact 
with the air spaces, and ultimately with the substomatal 
cavities, so that the mechanism is a physical system per- 
mitting diffusion. The protoplasm of each cell is thor- 
oughly penetrated with water; it is in contact with the 
penetrable cell-wall, an imbibition membrane, which is 
therefore moist. From such moist membranes water- 
vapor passes into the. intercellular spaces, which have a 
tendency to become saturated. Under external conditions 
favorable for evaporation there is a high gradient with 
respect to the external air, so that water-vapor diffuses 
rapidly from the substomatal cavity through the stomata. 

As a result of the work of Brown and Escombe it is clear 
that the stomatal system in such a plant as the sunflower, 
for example, constitutes an extremely efficient multiper- 
forate septum, the form and distance apart of the stomata 
commonly permitting, when they are open, a diffusion 
almost as rapid as though there were open space. This 



90 Plant Physiology 

is a fact of peculiar interest. Furthermore, it has been 
calculated that the capacity of the stomata in the sun- 
flower, for example, is about six times as great as any 
observed transpiration ; that is, the stomata only one 
sixth open would be sufficient to accommodate the most 
rapid loss of water which has been observed. 

The stomata exhibit a considerable range in size, but 
according to Eckerson the average approximates 18 x6/x. 
This minute size is scarcely appreciated until one com- 
pares it with some visible perforation, such as a needle- 
prick made with the smallest sewing needle, which is rela- 
tively enormous, measuring about 600 /x in diameter. 
Nevertheless, the total maximal stomatal opening of an 
average leaf is approximately one nineteenth of the 
surface. 

53. Distribution of stomata. — While stomata may 
occur in the epidermis of any plant organs, they are com- 
monly confined to the aerial surfaces, and especially to 
the leaves, or to organs performing the functions of leaves. 
As a general rule, in fact, it may be said that the under 
surfaces of the leaves are the situations most important 
with respect to stomatal occurrence. Eckerson has found 
that only about two fifths of the common greenhouse 
plants possess stomata on the upper surfaces. Weiss 
and others have collected considerable data showing the 
relative abundance of the stomata upon the different sur- 
faces of dorsi-ventral leaves, from which the following 
examples may be suggestive : — 



Transpiration and Water Movement 



91 



Leaves with No Stomata on the Upper Surfaces 



Plant 



Stomata per Sq. Mm. 
Lower Surface 



Abies balsamea (balsam fir) 
Acer pseudoplatanus (Norway maple) 
Anemone nemorosa (wind anemone) 
Begonia coccinea (red begonia) . 
Berberis vulgaris (barberry) . 
Ficus elastica (rubber plant) . 
Juglans nigra (black walnut) . . 
Lilium bulbiferum (lily) .... 
Morus alba (white mulberry) 
Ribes aureum (red currant) . 
Syringa vulgaris (lilac) .... 
Tropceolum majus (nasturtium) . . 



228 

400 

67, 

40 

229 

145 

461 

62 

480 

145 

330 

130 



Leaves with Stomata relatively Scarce on Upper Surfaces 



Plant 

Asclepias incarnata (milkweed) . 
Cucurbita Pepo (pumpkin) . 
Lycopersicum esculentum (tomato) 
Phaseolus vulgaris (bean) 
Populus dilatata (poplar) . . . 
Solanum Dulcamara 



Lower 


Upper 


SUBFACE 


Surface 


191 


67 


269 


28 


130 


12 


281 


40 


270 


55 


263 


60 



Leaves with Stomata more nearly Equal on Both Surfaces 



Plant 



A vena sativa (oats) . . . . 

Brassica oleracea (cabbage) 
Helianthus annuus (sunflower) 
Pinus sylvestris (pine) 
Pisum sativum (garden pea) 

Zea mays (corn) 



Lower 
Surface 


Upper 
Surface 


J 23 


|25 


'27 


Us 


301 


219 


325 


175 


71 


50 


216 


101 


| 158 


j 94 
1(52) 


1 (68) 



92 



Plant Physiology 



Leaves with More Stomata on Upper Surfaces 



Plant 


Lower 


Upper 


Nymphcea alba (water lily) 

Pinus strobus (white pine) 

Triticum sativum (wheat) 




14 


460 
142 
33 



54. The effects of conditions upon stomatal production. 
— It has been indicated that the preceding data are sug- 
gestive. They do not, however, represent absolute rela- 
tions, for the reason that the number of stomata is to a 
certain extent a factor of complex environmental condi- 
tions, varying with moisture-content of air or soil, light, 
temperature, and other conditions. In this connection 
some previously unpublished data l for corn and wheat 
grown under conditions similar except as to moisture-con- 
tent of the soil may serve as an illustration. The plants 
were grown in tumblers of fine sand for seventeen days, 
and the counts of stomata are averages for the microscopic 
field employed (8 x ocular and 16 mm. objective). 



Corn 


Wheat 


Per Cent 
of Water 
in Sand 


No. Lvs. 


Avg. Wt. 


No. 


Avg. Length 


Av-. Wt. 


No. 


per Plant 


of Tops 


Stomata 


of Tops 


of Tops 


Stomata 


38 


3.0 


3.63 


181 


3.9 


.46 


103 


30 


3.0 


3.54 


130 


6.4 


1.09 


85 


20 


3.0 


3.36 


129 


4.7 


.57 


82 


15 


2.8 


2.35 


124 


3.7 


.35 


81 


11 


2.0 


1.56 


107 


3.7 


.47 


59 



1 These data are the results of experiments made by Mr. F. M. Harris 
in my laboratory. 



Transpiration and Water Movement 



93 



The preceding table is sufficient to indicate that the 
number of stomata in a given area is variable. Again, 
there is no constant relation between the number in a 
given area and the size of the leaf ; for, in the highest and 
lowest moisture-content with wheat, both length of 
leaves and entire weight of tops are approximately 
equal, yet in the low moisture-content there are only 57 
per cent of the stomata found in the high moisture. 




Fig. 27. Stoma of Helleborus : position of guard cells open (darker 
lines) and closed (lighter lines) ; cell contents shaded. [After Schwen- 
dener and Strasburger.] 

55. The control of water-loss by stomatal movement. — 
The mechanism of stomatal movement has been abun- 
dantly studied. Commonly, wilting releases the tension 
which forces the guard cells apart, so that closure must be 
effected. Turgor of the guard and other cells of the 
stomatal region are therefore primarily important in 
determining the extent of the opening. The relative 
positions of the guard cells open and closed are shown in 
Figure 27, after Schwendener. 

Recent studies upon the relation of stomatal movement 
to transpiration point to several suggestions and conclu- 



94 Plant Physiology 

sions of interest, although these studies also make it evi- 
dent that there is yet much room for quantitative work 
in this field. In general, it may be said that, contrary to 
many early opinions, the stomata do not open and close in 
direct response to the varying conditions of the atmosphere 
which may inhibit or promote transpiration. When the 
plant is provided with sufficient moisture, the stomata are 
commonly open, but as Brown and Escombe and others 
have shown, maximum transpiration does not necessarily 
correspond with maximum opening. Wilting effects a 
closing of the pores, but according to Lloyd there can be 
no closing in anticipation of wilting. Again, the stomata 
may remain open when the humidity is extremely low, 
provided only sufficient water is available for the plant. 

Many investigators have shown a primary relation be- 
tween stomatal opening and the time of day; and it is 
believed that possibly through the possession of chloro- 
phyll, and the relation to organic food-materials, there 
may be found in the turgor of these cells an important 
factor in stomatal regulation. 

56. Modifications tending to check excessive transpira- 
tion. — Closure of the stomata is in all cases a means of 
checking the excessive transpiration, as already discussed. 
However, this check may be insufficient in extreme cases. 
It may prove also a menace to other activities of the plant. 
In any event many plants exhibit a structure peculiarly 
fitted to limit excessive transpiration. This is important 
in the occupancy by plants of arid habitats, and it is 
certain that many delicate species are unable to survive 
under conditions necessitating the most excessive trans- 
piration. This may be due in part to the incapacity on 



Transpiration and Water Movement 



95 



the part of such plants to respond to these conditions by 
the production of a protected surface or of growth-forms 
tending further to reduce the water-loss. 

When transpiration is excessive, leaves commonly droop. 
This is an indication of wilting. It is usually regarded as 
a further protection against water-loss. At all events, such 
leaves obey an obvious physical law. The leaves of corn 




Fig. 28. Stomatal apparatus in leaf of carnation. 

and some other plants " roll " under the same conditions. 
Here the tensions are different. It is often stated that in 
corn this rolling results because the stomata occur on the 
upper surface, but the data previously cited indicate that 
corn may exhibit more stomata on the lower than on the 
upper surface. 

Plants which are commonly able to maintain themselves 
to advantage in arid situations may be modified in one or 
more of a variety of ways, some of which are as follows : — 



96 Plant Physiology 

(1) Reduced surfaces, such as in cactus, aloe, and many 
desert plants. 

(2) Reduction in number of stomata, as in many grasses 
and sedges. 

(3) Sinking of stomata in special epidermal cavities as 
in yucca and carnation. 

(4) Thickened cuticle, as in carnation, pine, many desert 
plants, and the like. 

(5) Production of a waxy bloom upon the cuticle, as 
in cabbage, sugar-cane, and wheat. 

(6) The development of hairs upon the leaves, as in 
mullein and numerous mountain plants. 

(7) The possession of water-storage tissues, as in many 
desert plants, begonia, etc. 

57. Conditions affecting transpiration. — The condi- 
tions of the atmosphere greatly affect the evaporation from 
a water surface or from any other surface. In dry, hot 
weather, hay is quickly made and cured. The " pull " 
of the atmosphere upon all 
moist surfaces results, there- 
fore, in a prompt loss of water. 
In the same way transpira- 
tion in a healthy plant is ob- 
viously influenced by condi- 

From a stalk of sugar tioilS ° f the air > and [t is to a 

cane, epidermal region and certain extent influenced by 

"bloom" as a columnar deposit. j-a- r < i -i t 

[After De Bary.] conditions of the soil. In gen- 

eral, the important air fac- 
tors are humidity, temperature, wind velocity, and light. 
Low humidity, high temperature, rapid movement of the 
wind, and intense light commonly facilitate transpiration 




Transpiration and Water Movement 97 

to a marked degree. In fact, if the water-supply is not 
abundant, a combination of these conditions may promptly 
result in wilting. The water-loss is not necessarily pro- 
portional to changes in conditions, since when transpira- 
tion becomes excessive, concentration of the cell-sap and 
the closure of the stomata exert, in many cases, a most 
important inhibiting effect, which is, in a way, protective. 
The soil factors indirectly important in transpiration 
are water-supply and the strength or composition of the 
soil solution. 

58. Effects of excessive evaporation. — The permanent 
effects of an excessive loss of water vary with the type 
of plant. Herbaceous annuals might quickly wilt and 
dry up. Deciduous perennials might be promptly de- 
foliated. Many trees will show this during a summer 
drought, and if later wet weather prevails, there may be 
an entirely new season of growth. It is then comparable 
in effect to cold. Doubtless the excessive shedding of 
young flower-buds or " squares " of cotton is due to 
changes of the water relation. In general, reduced water- 
supply has a tendency to ripen up all parts, to mature 
seeds early, and often a considerable effect upon the com- 
position of the product. Seeds ripened in this wa}' are 
said to show the effects of immaturity, as shown later. 

59. Guttation. — The elimination of water as liquid 
may occur in certain plants when absorption is promoted 
and transpiration checked. It consists in the forcible 
excretion through certain stomata of water which may 
collect as drops on the edges of the leaves or may stream 
down the leaf blades. It is convenient \y observed upon 
young corn, blackberry, canna, and other plants. It may 



98 



Plant Physiology 



occur during a cool afternoon or evening of a warm day ; 
and quite commonly in the cool early morning after a hot day 




[Photograph by Russell and Harding.] 
Fig. 30. Destruction of cabbages by Pseudomonas campestris, a germ 
entering through the water-pores. 

which has served to heat up the soil to a considerable depth. 

The continuous water connection between the tissues 

and the external atmosphere established through guttation 



Transpiration and Water Movement 



99 



makes possible the entrance of the germ of the black-rot 
disease to the cabbage and allied plants. This organism 
is productive of one of the severest of the cabbage diseases 
(Fig. 30). 

60. Transpiration and evap- 
oration. — Since transpiration 
is an evaporation phenomenon, 
it is possible to compare the 
amount of evaporation in dif- 
ferent habitats, and thus be able 
better to determine or forecast 
plant behavior in such habitats. 
There are many difficulties in- 
volved in employing as a meas- 
ure the evaporation of water 
from a freely exposed water- 
surface. The simple evapo- 
rimeter devised b}^ Livingston 
is extremely satisfactory for this 
purpose (Fig. 31). 

This instrument affords a 
means of measuring the evap- 
oration from a porous cup. It 
consists merely of a 
bottle, or mason jar, 
through the well- 
paraffined stopper of 
which passes a tube 
connected by a rub- 
ber stopper with some 
type of porous cup 




Simple evaporimeter, Livingston 
form. 



100 Plant Physiology 

or filter tube, the whole being filled with water. This 
.filter tube may be shellacked to a known surface, adopted 
as a unit, and all other instruments may be standardized 
with respect to this. 1 Under different conditions the 
curve of water-loss from this instrument may not be 
comparable to that from a free water-surface; but the 
effects of conditions upon it are supposed to be more 
nearly comparable to the effects upon plant surfaces. 

The evaporimeter has also been serviceable in contrast- 
ing transpiration and evaporation in unit areas, thus 

relative transpiration may be taken as the ratio of trans- 
it 

piration to evaporation, conveniently expressed as — . The 

E 

extent of variability respecting this ratio has been regarded 
a fair indication of what is conveniently termed physio- 
logical checking of transpiration. In studying this regu- 
latory check upon transpiration Livingston finds that in 
certain desert plants it is especially operative between 
6.30 a.m. and 1 p.m., and especially pronounced at tem- 
peratures from 79 to 90° F. 

A proper study of the relation of certain horticultural 
plants to evaporation factors promises to yield much data 
of practical value. 

Various observers have compared the loss of water from 
leaves with the loss from an equal surface area of soil. 
Nobbe has shown that evaporation from the surface of 
the soil may be 1.6 to 5 times the amount lost from an 
equal leaf area. In these cases the ordinary crop plants 

1 Transeau (Bot. Gaz., 49:459, 1910) has recently constructed an 
instrument which seems to possess some advantages in the way of sim- 
plicity and rate of evaporation. 



Transpiration and Water Movement 101 

are considered. If, however, we should compare evapora- 
tion with the loss of water in such a plant as prickly pear 
(Opuntia), the former might be one hundred times as great. 
On the other hand, it should be remembered that a crop 
on a given plot may develop in leaf surface an area many 
times that of the soil upon which it is growing. Practi- 
cally all direct measurements of the relative water-content 
of bare soil as contrasted with areas producing crops 
indicate that the percentage of water-loss is greater where 
a crop is grown. In other words, it is possible to conserve 
water in the soil by fallowing. The use of a fallow, to- 
gether with sufficient cultivation to keep a constant surface 
mulch, is one of the first principles in dry-land farming. 

King 1 has cited a case showing the effects of fallowing 
versus cropping, which is striking. Two plots which had 
been almost identical in water-content were used in the 
experiment. After the summer fallowing there was the 
next spring in the upper surface foot 9.35 pounds per 
square foot (or 203 tons per acre) more water than in the 
soil cropped the previous season. A considerable differ- 
ence was still manifest after both plots had been cropped 
alike the succeeding season. 

Practically, therefore, plants deprive the soil of moisture. 
It is well known that willows or birches in a moist spot in 
a yard or meadow keep the soil fresh and mellow. In 
some cases trees or other vegetation may seem to increase 
the soil moisture, but a closer examination will generally 
reveal the fact that in such instances the vegetation pre- 
vents rapid run-off and, therefore, appears to use the 
smaller quantity. 

1 King, F. H., "The Soil," pp. 291-292. 



102 Plant Physiology 

61. Transpiration and growth. — It has long been evi- 
dent that there is, under certain circumstances, a relation 
or fairly definite ratio between transpiration and growth. 
As a result of various series of water cultures with wheat 
and other grasses, Livingston has attempted a further 
analysis of this relationship. He finds that the transpira- 
tion data are frequently as instructive as a comparison of 
total increase in weight or growth. It is observed, then, 
that transpiration and relative growth vary with weight 
and area of the leaves. The amount of transpiration is 
regarded as a simple function of the leaf surface, which 
again varies directly with leaf weight, or, practical^ speak- 
ing, with the weight of the entire tops. It follows, of 
course, that total transpiration is a more or less accurate 
measure of the total growth. 

This relationship, however, is limited by several factors. 
It is necessary to have conditions favorable for fairly rapid 
transpiration and favorable for growth. Again, increasing 
the salt content of the solution in which plants are grown 
measurably affects transpiration and may not increase 
growth materially, so that plants growing in diverse 
concentrations may show extreme variations with respect 
to the amount of water-loss. Reed has also recently 
demonstrated that potassium in any combination exerts 
a depressing effect upon transpiration, while a small 
quantity of tannic acid facilitates it. In other words, the 
relation applies to a relatively narrow set of conditions. 

62. Water transport. — This is a convenient but 
scarcely an accurate expression, since, except in the dif- 
fusion of water-vapor and in the formation of ice-crystals, 
there is, perhaps, within the plant no such thing as the 



Transpiration and Water Movement 103 

movement of pure water. In osmotic transfer water does 
move independently of substances in solution, but it is 
always associated with substances in solution. 

The movement of water in the plant has been a line of 
experimental inquiry since the dawn of plant ph3\siology. 
Many important facts have been clearly enunciated and 
numerous interesting data accumulated, yet some of the 
phenomena of movement observed find as yet no entirely 
satisfactory explanation. A chief source of difficulty lies 
in the complexity of the factors involved. 

In general, however, diffusion is important, but the rise 
and maintenance of water are complicated by such factors 
as capillarity, the cohesive strength of water columns, 
the lifting power of evaporation, the peculiar structure 
of the conducting vessels, and, under certain conditions, 
the existence of high root pressures. 

It has been stated that in the root the region of hair 
production is commonly characterized by a radial bundle 
arrangement apparently permitting more readily the move- 
ment of water from the parenchjmial cells directly into the 
woody portion of the bundle. There is, of course, no such 
thing in plants as a true circulation, analogous to the circu- 
lation of the blood in animals; yet it is possible to dis- 
tinguish in a very general way two types of movement in 
vascular plants, as indicated below. 

(1) There is a " transpiration stream," from the absorb- 
ing organs (of water containing some salts and com- 
monly traces of organic substances) to the leaves. This 
stream is directed mainly through vessels, the xylem part 
of the woody bundles (Fig. 32). During this transfer 
there is, moreover, general diffusion to all parts requiring 



104 



Plant Physiology 



water, or possessing a sufficient tension therefor. From 
what has been said of water-loss and of the principles of 
diffusion it will be apparent that the type of movement 




Fig. 32. Cross-section of primary fibrovascular bundle of Ricinus : 
phloem (P), showing sieve tubes (S), companion cells (AC), parenchyma 
and sclerenchyma ; cambium (C) ; and xylem (X), showing especially 
vessels (V) and tracheids. [After Curtis.] 

here discussed is more than diffusion. Moreover, rapid 
movement is essential in order to supply the demands of 
transpiration, and it is this transpiration stream which 



Transpiration and Water Movement 105 



effects, for one thing, perhaps, a rapid distribution of 
absorbed mineral nutrients. 

On the other hand there is (2) a more gradual movement 
by diffusion of soluble organic materials, or " elaborated" 
foods, along the paths provided by the plasmatically con- 
nected sieve tubes (Fig. 33), from which general paths 




1M: : I! H 




n 



:m I: 

si. s ac s 

Ph 

Fig. 33. Longitudinal section of a bundle similar to the preceding. 
[After Curtis.] 



i44$Hmm 



organic substances pass, also by diffusion, into all cells 
where growth and differentiation are proceeding. At 
certain seasons, in many plants during the spring, the 
movement of organic material in the xylem part of the 
bundles is common. Usually, however, the distinction 
may be made that the dead vessels or xylem elements 
conduct a liquid which is more nearly the nutrient solu- 
tion absorbed from the soil, whereas the sieve-tube part 
of the bundle is primarily the path of diffusion for organic 
materials. All cells of the body — parenchyma, cortex, 



106 



Plant Physiology 



and the like — permit of diffusion, and in the end the 
demands of each cell govern the flow toward that cell. 

63. Fibrovascular bundles. — If freshl}^ cut (under 
water) shoots of the jewel- weed, sunflower, Indian corn, 
canna, or other convenient rep- 
resentatives of monocotylous and 
dicotylous plants are placed in 
a solution of a dye such as eosin 
or fuchsin, the stain will pass 
upward through the conducting 
system of the plant, and the 
paths of conduction may thus be 
made evident, although there is 
sometimes a slight lateral diffu- 
sion tending to obscure the defi- 
nite channels. 

It will be recalled that mono- 
cotylous plants are characterized 
by stems in which the vascular 
strands are commonly distributed 
in irregular manner throughout a 
ground t issue called 
parenchyma, as in corn 
or sorghum. A hand 
section will show at a 
glance this distribution 
of the bundles, and it 

Fig. 34. Vascular system of Clematis, ls als0 strikingly 
apical portion of the stem : longitudinal brought Out by break- 
view of stem and leaf trace bundles (A) ing a dry CQm gtalk 
and cross-section of internode (B). & 
[After De Bary and NageiL] through an intemode 




Transpiration and Water Movement 107 

and noting the strands. Examined microscopically a 
bundle exhibits in cross section the typical collateral 
arrangement. In this the phloem or sieve-tube part 
is outermost, the xylem therefore within, and both are gen- 
erally inclosed by a sheath of stereome or mechanical 
supporting tissue. There is, however, no meristem or 
cambium within the bundle, signifying a closed type. 
The irregular distribution of bundles in the stem usually 
precludes the formation of a ring of wood, and there is, 
moreover, no bark in the usual sense. These are matters 
of much physiological significance, both from the stand- 
point of growth and conduction. 

In dicotylous plants the primary bundles are arranged 
in a ring and are also commonly collateral. The inter- 
position of secondary bundles (as subsequently discussed, 
section 187) may result in the formation of a complete 
wood-ring. Where no wood-ring is produced, the bundles 
run parallel throughout much of the internode (Fig. 34), 
divide and unite in a characteristic fashion at or near all 
of the nodes, also sending off branches to the leaves at 
each node. 

When a wood-ring is produced in a dicotylous stem, the 
meristem of the bundles forms a continuous growing layer, 
the outer portion of the wood-ring then consists of phloem 
and the inner portion of xylem. The cambium between 
permits the addition of seasonal or growth rings of phloem 
and xylem on the outer and inner sides respectively. In 
plants which attain a considerable age the parenchyma 
accompanying the bundles loses its protoplasm and the 
bundles cease entirely to take part in conduction. The 
number of rings of new wood which may be active in con- 
duction varies greatly with different plants. 



108 



Plant Physiology 



64. Leaf venation. — Each leaf receives a definite quota 
of bundles, and these and their subdivisions continued into 
petiole and lamina constitute the so-called venation sys- 
tem. In the case of monocotylous plants the veins are 
usually parallel from the leaf stalk, or from the mid-vein, 
so that they are often designated parallel- veined plants.. 

In the leaves of the dicotylous type the bundle sj^stems 
branch repeatedly, and also form a complete reticulum. 




[Photograph by H. M. Benedict.] 
Fig. 35. Minute venation of the leaf of Vitis riparia ; leaves of differ- 
ent ages. 

In any event the leaves are well provided with fibrovascu- 
lar tissue, easily demonstrated by macroscopic or micro- 
scopic observation. As a matter of fact the bundles extend 
to the most remote parts, and in dicotylous plants espe- 
cially the leaf is divided up into a complete network, with 
the areas between the vascular tissue being seldom larger 
than l-Jmm. in diameter (Fig. 35). The ultimate sub- 
divisions of the bundles consist of tracheids and elongate 
parenchyma cells (meristem). Sometimes the bundles 
end abruptly or blindly. As the leaf grows each area 
subtended by veinlets becomes larger, and this increase 



Transpiration and Water Movement 109 

in size may be followed by the laying down of new veinlets 
of a lower order (at first fewer tracheids) from each side 
of the original space. These may be at first procambial 
in nature, but tracheids are rapidly differentiated within. 
It is of special interest to note that the sieve tubes disap- 
pear relatively early in the minute continuations of the 
bundles. 

65. Rate of transport. — The rate of transport of water 
in the fibrovascular bundles may be determined with a 
fair degree of accuracy by means of the rise of dyestuffs 
as before noted, but more accurately in many cases by the 
method of Sachs, wherein lithium nitrate is used in the 
solution and its presence after intervals determined by 
burning the tissues and examining the flame spectro- 
scopically. According to Sachs the rate of water rise is 
extremely diverse, and may vary from a few centimeters 
per hour to one or more meters. Doubtless the extremes 
are often greater than these indicated, but unquestionably 
the difficulties of measurement are greater at the extremes. 

LABORATORY WORK 

Indication of transpiration. — Stahl's cobalt test may be 
employed to determine water-loss from a plant surface. In- 
cidentally it determines roughly the presence or absence of 
stomata, or the relative abundance upon the upper and lower 
surfaces of the leaf. Soak filter paper in a 5 per cent solution 
of cobalt chloride, dry in the oven or over a flame, and note the 
. blue color. Breathe upon a small piece of this paper and note 
that the absorption of moisture induces a change to pink. 
Now cut out two pieces of the paper of equal size ; place one 
upon the upper and one upon the lower side of the leaf to be 
tested, cover each with a piece of mica and cement the latter 



110 Plant Physiology 

to the leaf with plasticene or prepared wax. In this experiment 
handle the paper with forceps, and preferably use a leaf attached 
to the plant, or a shoot, the stem of which is immersed in water. 
Note any change of color, and the time required to produce 
change, in the two pieces. Experiment with several of the 
plants mentioned in section 53, and contrast your data with the 
indications regarding stomata there furnished. 

Amount of transpiration, determined by weight. — The actual 
transpiration of potted plants may be carefully determined by 
loss of weight, as already indicated. Employ plants of any 
kind convenient, preferably one-stemmed plants with large, 
relatively simple leaves ; inclose the pot in soft rubber cloth, in 
aluminium shells and rubber cloth, or in any manner convenient 
to prevent evaporation from the pot and soil, the plant being 
previously watered. Weigh carefully and repeat the weighing 
after each of several intervals of not more than twelve hours. 
If water is again applied, add approximately the quantity lost, 
and weigh again. Plot the results. This experiment may be 
extended through a considerable period of time, and different 
types of plants may be contrasted. Ultimately, the area of 
each plant must be taken into consideration or unit areas com- 
pared, as indicated in later experiments. 

The transpiration of plants grown in water cultures in par- 
affined wire baskets, or in glazed pots covered with paraffin, 
is also conveniently determined by weighing, as referred to in 
subsequent sections of this book. 

Measurement of leaf areas. — It would be difficult to deter- 
mine directly the transpiration of a tree or of any vegetation 
under natural conditions. For the laboratory experimental 
work in contrasting different plants or plants under different 
conditions, as well as for an indication helpful in estimating 
water-loss in the field, it is desirable to have a quick method of 
measuring leaf areas. 

Many methods of determining leaf areas are now used. Ordi- 
narily it is sufficient to trace the outline of the leaf upon coordi- 
nate paper, the area being determined by a count of spaces. 
Another simple method is to trace the outlines of the leaves 



Transpiration and Water Movement 111 

employed (or of average leaves) upon paper of uniform thick- 
ness, these outlines being subsequently cut out and weighed 
accurately for comparison with the weight of a known area. 
Prints may be made upon sensitized paper, or the planimeter 
may be employed. The area of stems and petioles is generally 
negligible, but may be roughly estimated when necessary. It is 
well to express all transpiration data, as suggested by Ganong, 
in grams per hour per square meter of surface, written g m % h. 

Amount of transpiration, determined by potometers. — Set up a 
transpiration experiment, employing either the Ganong potom- 
eter (Fig. 24), a burette potometer (Fig. 23), or some other 
form equally satisfactory. The former may be employed for 
short periods, contrasting the effect of conditions ; while the 
short burette potometer may be used for longer intervals, care 
being taken, however, to keep the column of water in the burette 
at a height approximately equal to that in the other arm. 1 

Effects of conditions upon transpiration. — It has been indi- 
cated that temperature, humidity, air movement, etc., are 
directly and indirectly important in varying transpiration quan- 
tities. While the effect of light variation may be demonstrated, 
more satisfactory experiments may be made with the other 
factors. 

A rough idea of the effects of temperature and humidity may 
be obtained by simple transpiration experiments with simul- 
taneous observations upon simple thermometers and hygrom- 
eters, placing plants of more or less equivalent areas, even for 
a few hours, under conditions determined to be diverse. With 
more or less equal lighting, contrast the transpiration, for ex- 
ample, in a moist basement room with that in a warm upper 
room ; or, at a uniform temperature, also, contrast a plant 
exposed in a quiet room with one in the same room under a large 
bell glass, the latter securing greater humidity. Diverse con- 

1 In all transpiration or other experiments where the further ab- 
sorption of water by excised shoots is required, the cutting of the shoot 
should be done while it is bent under water, and the ends of the shoots 
should be promptly immersed in water until used. 



112 



Plant Physiology 



ditions may also be obtained in different greenhouses. Keep 
an accurate record of the conditions of the experiments, and 
accompany this with a record of the transpiration data, express- 
ing the latter in terms of g m 2 h, as above explained. 

A more accurate evaluation of the factors, and consequently 
a better conception of the effects of external conditions upon 
plants, may be obtained by means of experiments continued 
several days, whilst utilizing, also, autographic recording in- 
struments. Study the mechanism of the thermograph (Fig. 112) 
and hygrograph (Fig. 36) ; also set up and standardize some 
simple evaporimeters (Fig. 31) after the method of Livingston 



ait****^-- iJa 




Fig. 36. 



[Illustration from Julien P. Friez.] 
Hygrograph. 



(section 60) or of Transeau (Bot. Gaz., 49: 459, 1910). Then 
set up in duplicate with the burette potometers a transpiration 
experiment (preferably two, under two sets of conditions). 
This is to be accompanied by the continuous recoroL of tempera- 
ture and humidity. Make observations upon water-loss as 
often as possible. The experiment may be continued several 
days if shoots with woody stems are chosen. Plot and di&cuss 
the results. \ 



Transpiration and Water Movement 113 

Under conditions otherwise similar place one plant (or potom- 
eter) and a standardized evaporimeter in a current of air (an 
electric fan may be employed), and a similar plant and instru- 
ment in a quiet atmosphere. Contrast the water-loss after a 
sufficient interval. 

Guttation. — Water freely some potted plants of cabbage or 
corn with warm water until the temperature of the soil is about 
35° ; then transfer the pots promptly to a cool room, cover 
with bell glasses, and after a few hours describe any exudation 
phenomena noted. 

Leaf structure. — Make hand sections of a variety of leaves 
(at least four types) and compare by microscopic study, es- 
pecially the epidermis, palisade tissue, and intercellular spaces. 
The following leaves are suggested : beech, cherry, or ivy ; 
rubber plant or rhododendron ; snap-dragon or jewel-weed ; 
carnation or small cereal ; pine or spruce. Draw in detail one 
type. 

Examine and compare, if possible, leaves of any variety of 
small cereal grown under diverse water conditions. Note es- 
pecially, the width and venation of the leaf, the amount of 
bloom, and the number and distribution of the stomata. 

Conduction of water. — Cut under water several shoots of 
young sunflowers, castor-oil plants, jewel-weed, corn, and some 
plants with light-colored flowers (hyacinth, phlox, or other her- 
baceous plant convenient), and place the cut ends in vessels 
containing a red dye. After the lapse of an* hour or two note 
the course of the dye through the stem, also into the leaves and 
petals. With long standing is there more general diffusion of 
the dye ? Describe the results. 

Remove shoots which have been in the dye for a very short 
period (15 minutes to 1 hour), wipe off the surplus dye from 
the outside with filter paper, and with a sharp knife or razor 
cut off the stem and examine promptly with the hand lens to 
ascertain what portion of the bundle is colored. In the case 
of the sunflower and castor-oil plant is the entire ring colored ? 
Peel off the bark of the dicotylous plants and examine it for 
the dye. Draw conclusions. 



114 Plant Physiology 

The rate of movement may be studied by leaving the shoots 
from half an hour to one hour in the dye, then cutting off the 
stems at successive intervals until the uppermost indication of 
the stain is found, through examination with a hand lens. After 
determining the rate of rise of the liquid at laboratory tempera- 
ture, place some shoots under conditions favorable for rapid 
transpiration and others under a bell glass, and contrast the 
results. According to the directions in the next paragraph l de- 
colorize a leaf of the grape, sunflower, or fuchsia, and under the 
low power of the microscope study the minute ramifications of 
the veins. 

Place fresh tissue in equal parts of 95 per cent alcohol and 
glacial acetic acid. After from 24 to 48 hours, take pieces and 
hold them immersed in pure nitric acid until clear (usually a 
matter of seconds), place on a slide, add glycerin, and boil 
over flame until tissue becomes entirely transparent. Put on 
cover glass and examine in the glycerin. 

Ring small plants such as geranium, sunflower, Ricinus, or 
other forms with definite bark, by removing a circle of bark 
about one fifth of an inch long, extending completely around 
the stem. The plants should not be in an atmosphere so ex- 
treme as to cause rapid drying-out from the cut surface. Does 
ringing interfere with the conduction of water to the leaves? 

References 

Burgersteix, A. Die Transpiration der Pflanzen. 142 pp., 

14 pis., 1904. 
Clapp, G. L. A Quantitative Study of Transpiration. Bot. 

Gaz. 45 : 254-267, 32 figs., 1908. 
Clements, E. S. The Relation of Leaf Structure to Physical 

Factors. Trans. Am. Mic. Soc. (1905) : 19-102, 9 pis. 
Copeland, E. B. The Rise of the Transpiration Stream. Bot. 

Gaz. 34 : 161-193 ; 260-283, 1902. 

1 A method suggested by Professor H. M. Benedict, University of 
Cincinnati. 



Transpiration and Water Movement 115 

Darwin, F. Observations on Stomata. Phil. Trans. Roy. Soc, 
London. 190 B : 531-621, 1898. 

Dixon, H. H., and Joly, J. On the Ascent of Sap. Phil. 
Trans. Roy. Soc. 186 : 563-576, 1895. 

Eckerson, S. H. The Number and Size of the Stomata. 
Bot. Gaz. 46 : 221-224, 1908. 

Ewart, A. J. The Ascent of Water in Trees. Phil. Trans. Roy. 
Soc. 198 B : 41-85, 1906. 

Harter, L. L. The Influence of Soluble Salts upon Leaf Struc- 
ture and Transpiration of Wheat. Bur. of Plant Ind. Bui. 
134 : 22 pp., 1908. 

Livingston, B. E. The Relation of Desert Plants to Soil 
Moisture and to Evaporation. Carnegie Institution Publ. 
50:78 pp., 16 figs., 1906. 

Lloyd, F. E. The Physiology of Stomata. Carnegie Institu- 
tion Publ. 28 : 142 pp., 40 figs., 14 pis., 1908. 

Reed, H. S. The Effects of Certain Chemical Agents upon the 
Transpiration and Growth of Wheat Seedlings. Bot. 
Gaz. 49 : 81-109, 9 figs., 1910. 

Texts. Barnes, Detmer, Ganong, Goodale, Jost, Pfeffer, Stevens. 



CHAPTER VI 

THE WATER REQUIREMENTS OF CROPS 
AND OF VEGETATION 

From what has been said respecting the use of water by 
plants, more especially transpiration, it is obvious that 
the requirements of vegetation and of crops will be most 
diverse, and that any particular crop or type of vegeta- 
tion will show a modified use dependent upon temperature, 
light intensity, strength of soil solution, texture of soil, 
and the like. 

66. Relative requirements of a few crops. — Lyon and 
Fippin have compiled a statement of the water needs of 
several crops which is suggestive. These crops were tested 
by the different observers under dissimilar conditions, and 
close agreement is not to be expected. Moreover, the 
methods of controlling or estimating the evaporation of 
water from the surface of the soil has not been the same 
with the different observers, and this might easily lead to 
important differences. See table on opposite page. 

Taking 300 pounds of water as an average amount 
transpired by crop plants, in order to produce 1 pound of 
dry matter under conditions in England, Hall has prepared 
a table giving the precipitation necessary to supply the 
water used by certain crops. For conditions in the central 
116 



Water Requirements 



117 



Water transpired by Growing Plants for One Part of Dry 
Matter Produced 



Estimations made by 



















Hellriegel 




Wollny, 




King, 


England 




Germany 




Germans 




Wisconsin 


Beans 


214 


Beans 


262 


Maize 


233 


Maize . 1 272 


Wheat . 


225 


Wheat 


359 


Millet . 


416 


Potatoes . | 423 


Peas . . 


235 


Peas . . 


292 


Peas . . 


479 


Peas . . 447 


Red clover 


249 


Red clover 


330 


Rape . 


912 


Red clover 453 


Barley . 


262 


Barley . 


310 


Barley . 


774 


Barley . 


393 






Oats . 


402 


Oats . . 


665 


Oats . . 


557 






Buck- 




Buck- 












wheat . 


371 


wheat . 


664 










Lupine . 


373 


Mustard . 


843 










Rye . . 


377 


Sunflower 


490 






Average 


237 


341 




608 




424 



United States it would be more nearly accurate to assume 
an average requirement according to King's results of 
about 425 pounds of water for each pound of dry matter. 
Modifying the data in accordance with this, the indications 
are as follows : — 









Wt. of 






Crop 


Wt. at 


Per Cent 


Dry Mat- 


Calc. Water used 




Harvest 


of A\ A.TEB 


ter at 
Harvest 


during 


Growth 




Tons per A. 




Tons per A. 


Tons per A. 


In. of rain 


Wheat . . . 


2.5 


IS 


2.05 


922.5 


9.13 


Barley . . . 


2.0 


17 


1.66 


747 


9.39 


Oats .... 


2.5 


16 


2.10 


945 


9.36 


Meadow hay 


1.5 


16 


1.26 


567 


5.61 


Clover hay . . 


2.0 


16 


1.68 


756 


7.48 


Swedes . . . 


17.0 


88 


2.04 


918 


9.09 


Mangolds . . 


30.0 


88 


3.60 


1620 


16.03 


Potatoes . . . 


7.5 


75 


1.87 


841.5 


8.32 


Beans . 


2.0 


17 


1.66 


747 


7.41 



118 Plant Physiology 

In all cases the amount of water given is a considerable 
part (averaging about one fifth in the central United 
States) of the annual rainfall. Considering the run-off 
and the evaporation from the soil, both during and outside 
of the growing season, it is essential to study carefully 
the water requirements of crops, even in regions where 
the rainfall seems generally adequate. 

An abundant or optimum supply of water in part 
obviates the necessity of maximum cultivation, since 
cultivation may be very considerably concerned with con- 
servation of water. Nevertheless, there are factors of aera- 
tion, proper conditions for certain types of bacterial action, 
texture of soil, and the like which require cultivation, 
wholly aside from the water relation. 

67. Precipitation and crop growth. — Under ordinary cir- 
cumstances the greater part of the precipitation water 
is not conserved by the soil. Of the total annual rainfall 
only a certain percentage is available to vegetation or to 
the crop. Some of the water is lost in the immediate 
surface run-off, a small part may be lost by percolation, 
and there is further a considerable amount represented by 
evaporation. When the water-table is low, plants are, of 
course, wholly dependent upon the water which is con- 
served in relatively superficial strata. 

Practically speaking, no section of Europe or of the 
United States is wholly free from droughty periods. This 
implies the well-recognized fact that precipitation during 
the growing season is demanded by the great majority 
of crops and types of vegetation. Nevertheless, when 
proper measures are taken for the conservation of water 
which may fall outside of (as well as during) the growing 



£J2 __g_S o 7 w o <* in (D + p. 

-x f -t^ ,- -' J - "" ^ "o "o "o "o o : o b \ ^ j 




120 Plant Physiology 

season, a relatively small precipitation — say 25 to 30 
inches — may be sufficient for crop production. Early 
maturing grains and other grasses require as little, perhaps, 
as any other type of vegetation affording an equal yield. 

A chief cause of the annual variation in yield of many 
staple crops is to be found in the variation in rainfall. 
Smith has prepared charts showing a remarkable agreement 
between yield of corn and precipitation in the corn belt 
of the United States for the chief growing months — June, 
July, and August. In Figure 38 the dotted line gives 
the average rainfall for the months mentioned, covering a 
period of fifteen years, and the full line gives yield of grain 
per acre for the same time. The data are taken from 
Ohio, Indiana, Illinois, Iowa, Nebraska, Kansas, Missouri, 
and Kentucky. 

The chart (Fig. 37) shows a rainfall map of the United 
States for a period including, in the main, the growing 
season. From this, it is apparent that the rainfall west of 
the hundredth meridian practically to the Coast Range 
valleys of the Pacific is less than the usual requirement, 
and so the number of crops which may be grown in this 
general region without irrigation is extremely limited. In 
fact, throughout a very large portion of western North 
America, eastern and southern Europe, northern Africa, 
and a large part of Asia and Australia, crop production is 
limited much more by insufficient or ill-distributed rain- 
fall than by all other factors combined. 

It is believed that the great agricultural countries of 
the world must be, in time, those of great area, such as 
Australia, Brazil, China, India, Russia, and the United 
States. In three of these, however (China, India, and 



Water Requirements 



121 




122 Plant Physiology 

Russia), famines may be expected any year when the rain- 
fall is but slightly less than usual, and without their fairly 
well-developed systems of irrigation much larger areas of 
these countries would remain in doubt with respect to pro- 
duction. Where irrigation is not practiced, it is frequently 
necessary to introduce systems of dry-land farming 
whereby the principles of soil-moisture conservation are 
effectively applied, and sometimes a single crop is grown 
in two years, water being allowed to accumulate every sec- 
ond year. 

Precipitation has a maximum effect, of course, when all 
conditions are favorable; that is, when the nature of the 
soil and its depth, the type of sub-soil, the slope and ex- 
posure of the ground, all combine to conserve moisture 
and deliver it to the growing crop. 

68. Irrigation. — Both in Europe and America (in 
many sections where irrigation has not been considered 
necessary) it has now been abundantly demonstrated that 
the yield of most crops may be materially increased by a 
rational use of water. In Wisconsin, for example, King x 
has found that during a six-year period the yield of pota- 
toes was increased from 217.3 bushels to 301.7 bushels 
per acre. Again, with twelve inches of rainfall during a 
growing season for corn, the yield of grain was increased 
by means of irrigation from 30.14 to 65.3 bushels. 

It has already been indicated that profitable crop pro- 
duction is only possible in many regions when irrigation 
practices supplement the effect of the normal rainfall. 
In every drainage basin, large or small, there are oppor- 
tunities for conservation. 

1 King, F. H., Wis. Agl. Exp. Sta. Report, 18 : 195, 1901. 



Water Requirements 



123 






■ - re i ST - TStf-^ \J < 









- 








Rice-field prior to drawing off water for harvesting, 
Louisiana. 



Fruits. — In most sections of the Pacific coast region, 
deciduous fruits are commonly irrigated when the rainfall 
is less than twenty inches, and many believe that irriga- 
tion may be desirable when the precipitation is equal to 
or somewhat greater than this amount. Citrus fruits 
grown on a commercial scale in that region are invariably 
irrigated. In all cases the purpose of irrigation, as Wick- 
son says, " is a means of soil improvement to be emploj^ed, 
like other means of improvement, when the soil needs 
it." 

The following tables will suffice to indicate for the two 
types of fruit mentioned the usual amount of water added 
by irrigation to supplement the normal precipitation : — 



124 



Plant Physiology 

Deciduous Fruits 





Rainfall 
in In. 


Irrig. Season 


No. 
Irrig. 


Each 
Irrig. In. 


Total 
Irrig. In. 


Sacramento . . 


18-20 


June to October 


3-18 


1-1.25 


3.25-18 


Santa Clara . 


12-20 


Spring, summer, 
or winter 


1- 3 


3-12 


12-16 


Fresno . . . 


8 


Summer or win- 
ter 


1- 4 


2.5-12 


7.5-12 


Los Angeles . . 


12-20 


Spring or sum- 
mer 


1- 3 


2-9 


4-9 



Citrus Fruits 





Rainfall 


Irrig. Season 


No. 


Each 


Total 




in In. 


Irrig. 


Irrig. In. 


Irrig. In. 


Fresno . . . 


8 


April to October 


2-7 


2 


4-14 


Los Angeles . . 


10-20 


March to No- 
vember 


3-7 


1-9 


3-27 


San Bernardino . 


12 


March to De- 












cember 


4-8 


1.5-6 


6-36 


Riverside . . . 


7-12 


When needed, or 
April to De- 












cember 


3-9 


1.5-6 


10-36 


Orange . . . 


10-18 


When needed, or 
March to De- 












cember 


3-8 


2-5 


10-40 


Tulare . . . 


10 


March to Octo- 
ber 


5-10 


2.6 


12-60 



Corn. — The period of growth of this plant is long, 
and in temperate regions it extends throughout the 
warmest season. The water requirements are consider- 
able; consequently in semiarid or dry regions it responds 
abundantly to proper irrigation. The most striking re- 



Water Requirements 



125 



suits have been secured at the Utah Experiment Station. 
In the table below there are included the data respecting 
yield, and also the effect upon protein content : — 



Irrig. Water In. 


Yield of Grain per A., 


Protein in Water-free 


Applied 


Bu. 


Substance, Per Cent 


38.00 


82.71 


12.99 


36.53 


69.28 


12.05 


19.98 


77.00 


13.00 


19.97 


49.28 


12.65 


15.00 


46.28 


13.17 


15.00 


58.71 


13.79 


10.00 


56.86 


13.42 


7.50 


35.14 


15.08 


0.00 


26.00 


14.52 



Wheat. — In comparison with the data given for corn 
with different amounts of irrigation, it is of interest to 
examine the results secured at the same station with wheat. 
The accompanying table indicates not only the amount of 



Water 

Applied, 

In. 


Yield per 
Acre, Bu. 


Percent- 
age OF 
Protein 


Percent- 
age of Ash 
in Grain 


Yield (in 

Pounds) per 

Acre of 


Yield (in 

Pounds) per 

Acre of 




in Grain 


Nitrogen 


Ash 


4.63 


4.50 


24.8 


2.50 


10.7 


6.75 


5.14 


3.83 


23.2 


3.07 


8.5 


70.5 


8.73 


10.33 


19.9 


2.54 


19.7 


15.74 


8.89 


11.33 


19.4 


2.93 


21.1 


19.72 


10.30 


14.66 


18.4 


2.34 


25.9 


20.24 


12.09 


11.16 


21.3 


3.25 


22.8 


21.44 


12.18 


11.66 


23.1 


2.88 


25.8 


20.30 


12.80 


13.00 


17.1 


2.52 


21.3 


21.50 


17.50 


15.33 


17.2 


2.57 


25.3 


23.64 


21.11 


17.33 


15.9 


2.34 


26.4 


24.33 


30.00 


26.66 


14.0 


4.14 


35.8 


26.20 


40.00 


14.50 


17.1 


2.52 


23.8 


21.92 



126 Plant Physiology 

water supplied and the resulting yield, but also the pro- 
tein and ash content of the grain and the total amount 
of these components (protein as N) taken from the soil. 
From the preceding table it is evident that, in general, 




Fig. 40. Springs and reservoir for irrigation of date-palms, Figuig, 
Morocco. 

irrigation water up to thirty acre inches increased the yield 
of grain and diminished the nitrogen content. The effect 
of an increase of over thirty inches of water is greatly to 
diminish the yield; but the percentage, the composition, 
and the total removal of soil constituents per acre remain 



Water Requirements 127 

practically the same as when one third as much water 
was supplied. 

Date-palm. — In the Saharan region of northern Africa, 
where the date-palm is most extensively grown, the pre- 
cipitation is commonly less than ten inches. Moreover, 
during the growing season the air is intensely dry, and 
evaporation reaches a maximum. Under such conditions, 
and assuming no subterranean water-supply, it has been 
estimated l that this plant (a tree of medium size) requires 
a maximum of from 100 to 190 gallons of water per day 
during at least four months, making a total of from 3y to 
4 i feet of irrigation-water annually. 

69. Potted plants and water supply. — Potted plants 
possess such diverse water requirements that it is often 
difficult for the amateur grower to arrive at any satisfac- 
tory principles for watering. First of all, it is clear that 
the amount given should be in proportion to the water- 
loss. Plants in a dry room or greenhouse may require 
many times as much water as those in a shaded green- 
house full of vegetation, with air fairly saturated. Most 
potted plants are quickly injured or killed by constant 
saturation, and the practice of saturating the pot and filling 
the jardiniere around it is soon fatal; for with the usual 
amount of organic matter in the soil the exclusion of air 
to this extent is harmful both directly and indirectly. 

Viewing this matter in the light of such experimental 
work as has been undertaken, it seems that during the 
growing season a constant, favorable supply of water from 
below is most desirable. This, of course, is not always 

1 Swingle, W. T., " The Date-palm." Bur. Plant Ind., U. S. Dept. 
Agl., Bui. 53 (cf. pp. 47-48), 1900. 



128 Plant Physiology 

practicable, but it serves to emphasize the fact that alter- 
nate flooding and drying is not necessarily ideal. The 
latter is far better than stagnation. When in vigorous 
growth, the plant suffers from drying-out, and some part 
of the absorbing surface is killed every time the soil be- 
comes air-dry. 

In general, there is a certain relation between abundant 
water-supply and vegetative growth, so that it may be 
necessary to check watering somewhat to induce more 
abundant flowering. Again, in the case of plants which 
flower periodically, it may be desirable, or even imperative, 
to permit the plant to pass into a resting or semidormant 
condition. If the plant as a whole is to remain alive, water 
may not be entirely withheld, but in. the case of many 
bulbous and fleshy-rooted plants it may be highly desirable 
that all other vegetative organs disappear, and coinci- 
dently it may be desirable that all other conditions favor- 
ing metabolism (such as high temperature) may be reduced. 

The cultivation of plants whose peculiar growth-forms 
are dependent upon dryness of habitat is a special case, 
just as is the cultivation of water plants, and some of the 
general relations of these types are subsequently treated. 

70. Ecological classification based upon the water 
relation. — In the previous paragraphs of this chapter 
plants of the most diverse water relations have been dis- 
cussed ; those of the desert represent one extreme and 
those of ponds and water-courses the other, between which 
extremes falls the great majority of plants. The water 
relation w T as recognized by Warming to be most important 
in attempting a habitat or ecological classification of forms. 
With respect to this factor he has made from the natural 



Water Requirements 



129 




intergrading series of forms three primary groups which 
are conveniently designated (1) xerophytes, (2) meso- 
phytes, and (3) hydrophytes. 

Xerophytes. — Plants adjusted to physiological dryness 
are properly termed xerophytes. In the preceding pages 
references have been made to 
the fact that there are a large 
number of plants both peren- 
nial and annual which are able 
to exist in typical desert situ- 
ations. In general, such plants 
are tough, often hairy, and 
they usually possess reduced or 
leathery leaves. Accompanying 
these modifications there may 
be histological adjustments 
which may serve to check 
water-loss during the more arid periods, and to accu- 
mulate or store water when it is more plentiful. Special 
peculiarities of the epidermis, and of the plant in general, 
as affecting transpiration, have been discussed. The 
cactus, yucca, and sage-brush of the southwest are 
plants possessing the capacity for types of modifications 
which enable them to persist and to become the dominant 
vegetation in much of that region. 

Some of the most famous writing papers are those manu- 
factured in Scotland and England from a widely distrib- 
uted and much exploited African desert grass known as 
alfa. This name refers particularly to Stipa tenacissima, 
which occupies millions of acres in the steppes of northern 
Algeria. It is a plant too tough even to furnish food for 



Fig. 41. Section of Begonia 
leaf, showing colorless water- 
storage tissue adjacent to epi- 
dermis. [After Coulter.] 



130 



Plant Physiology 




Water Requirements 131 

the camel, and it thrives under conditions which would 
perhaps eliminate the great majority of the grass species 
of the semiarid United States. 

Mesophytes. — The mesophytes occupy an intermediate 
position with respect to water requirements. They con- 
stitute therefore the chief elements of the terrestrial flora, 
and in fact the main crops and herbaceous vegetation of 
the earth. Likewise the species constituting the typical 
forests of northern Europe and America, as well as most of 
those of tropical regions, would be classed in this category. 
In other words, we have in this group a great majority of 
those plants which constitute crops in the usual sense of 
this term. The relative abundance of plants requiring 
an intermediate amount of water results in a tendency 
to consider them as the normal plants, whereas others may 
be regarded as abnormal, or as specially adjusted to per- 
sistence under intensified conditions. 

Hydrophytes. — All plants growing wholly or partially 
submerged are denoted hydrophytes. Typical members 
of this group, such as the water lilies, or water millfoil, 
exhibit modifications of structure which arc of much 
interest. It is important to refer again to the fact that 
soil water is not pure, and must of necessity contain suit- 
stances in solution. The water of most streams, ponds, 
and inland lakes contains relatively small quantities of 
organic matter, and invariably small amounts of many 
mineral compounds; such fresh waters support one type 
of vegetation. Ponds which are common in the typical 
bog regions of the northeastern United States and else- 
where may contain organic materials produced under 
peculiar conditions, which apparently serve to make the 



132 Plant Physiology 

water less valuable physiologically. Any tendency in 
this direction results in a similar inclination to xerophyt- 
ism in the flora which may occupy these waters. From 
the standpoint of the relations of vegetation in general, 
the water of the sea is to be regarded as almost unavailable 
physiologically, on account of the large content of salt 
which it contains. Flowering plants which grow in salt 
marshes are often, therefore, typical xerophytes. 

71. Semi-xerophytism and hard-wheat production. — 
The hard-wheats are species or varieties which, for the 
perfection of their particular economic qualities, require a 
relatively small water-supply. They are the varieties now 
cultivated in much of the Central West immediately west 
of the hundredth meridian. In this section the precipi- 
tation during the growing season is so inadequate as dis- 
tinctly to shorten the growing period. This is, moreover, 
emphasized by the high temperature of the summer season. 
Other factors may play a part, but in general the growing 
season is determined by the conditions mentioned. 

This shortening of the growing season is apparently 
wholly comparable to incomplete maturity. The hard- 
wheats have a tendency to produce high nitrogen content, 
and immaturity accentuates the relative increase in pro- 
tein material and sometimes seems even to augment the 
total absorption of nitrogen. At any rate, in high nitrogen 
content, or gluten, lies the advantage of these wheats for 
semolina and other purposes (including bread-making) 
to which they are put, so that there exists an interesting 
relation of region to product. 

The hard-wheats have, for the most part, originated 
under conditions more or less similar to those prevailing 



Water Requirements 



133 



in the West, and the introduction of these varieties has 
greatly increased crop production and the possibilities of 




Fig. 43. Semiarid sandhills of eastern Colorado ; Andropogon scoparius 
(bunch grass) and Bouteloua Mr suta. [Photograph byH. L. Shantz.] 

agriculture. These wheats are apparently adjusted struc- 
turally to absorb more water, by increased root develop- 
ment, and to conserve it better, by lessened transpiration. 



134 Plant Physiology 

They also mature early, ripening before the conditions are 
such as to prevent development, and finally, they are able 
to adjust themselves more or less to considerable changes 
within the growing period. 

The following summary, adapted from Lyon, may 
therefore indicate the conditions under which hard-wheat 
production may be maintained : — 

(1) A relatively dry atmosphere which emphasizes the 
drought conditions. 

(2) A short growing period, which is equivalent to 
early maturity and is unfavorable to starch-storage in the 
later stages of growth. 

(3) A favorable nitrogen supply in available form. 

It may be readily inferred that if certain of these condi- 
tions do not naturally obtain, or if they are artificially 
changed, there may be a tendency to make softer wheats 
of the hard varieties. From experiments in Washington 
(Thatcher), it has been shown that the total precipitation 
in the different counties of the state governs very closely 
the composition of the kernel ; therefore, as under irriga- 
tion, there is here a tendency — with higher precipitation 
— to produce the characters of soft-wheat where otherwise 
a hard- wheat would be developed. 

SUBSIDIARY WORK 

Students not taking work directly along agricultural lines may 
be required to prepare a report upon some phase of the water 
requirements directly related to drainage, some aspect of irriga- 
tion, or the water-relations of special crops, utilizing any ac- 
cessible literature. Agricultural students, less likely to consider 
adequately the ecological aspects of vegetation, may be given a 



Water Requirements 135 

topic requiring some careful study in -Schimper and Warming, 
or a critical analysis of any special articles available. 

References 

King, F. H. Irrigation and Drainage. 502 pp., 163 figs., 1899. 
Lyon, T. L., and Fippin, E. O. Soils. Pp. 133-136, 1909. 
MacDonald, W. Dry Farming. 290 pp., 24 figs., 1909. 
MacDougal, D. T. Delta and Desert Vegetation. Bot. Gaz. 

38 : 44-63, 7 figs., 1904. 
Shantz, H. L. A Study of the Vegetation of the Mesa Region 

Eastof Pike's Peak. Bot. Gaz. 42 : 16-47, 7 .fas. ; 179-207, 

6 figs., 1906. 
Smith, J. W. Relation of Precipitation to Yield of Corn. 

Yearbook U. S. Dept. Agl. (1903) : 215-224. 
Volkens, G. Flora der agyptisch-arabischen Wiiste. 
Wickson, E. J. Irrigation, in Field and Garden. U. S. Dept. of 

Agl., Farmers' Bui. 138 : 40 pp., 18 figs., 1901. (Also Off. 

Exp. Stas. U. S. Dept. Agl. Bui. 108 : , 113 : etc.) 
Widtsoe, J. A., and McLaughlin, W. W. The Right Way to 

Irrigate. Utah Agl. Exp. Sta. Bui. 86 : 101 pp., 12 pis., 1903. 
Wilcox, L. M. Irrigation Farming. 494 pp., 113 .fas., 1902. 
Wollny, W. Untersuchungen iiber den Einfluss der Luftfeuch- 

tigkeit auf das Wachsthum der Pflanzen. Wollny's Forsch. 

a. d. Geb. d. Agrikult.-Physik. 20 : 397-437, 1898. 

Texts. Clements, Jost, Schimper, Soraner, Warming. 



CHAPTER VII 
MINERAL NUTRIENTS 

In a physiological sense the common fertilizers, or " arti- 
ficial " fertilizers, sold upon the markets of a large part 
of the world are, with respect to plants, soil nutrients or 
amendments. A stud}' of fertilizers and of conditions and 
factors governing the use of these under diverse field con- 
ditions constitutes a special phase of agronomic or soils 
work. At all points this field of work overlaps physiolog- 
ical inquiry, for ultimately the plant response, or yield, 
is the index to favorable or unfavorable soil condition. 
But the most significant fact is that in his most important 
work the agronomist usually deals with these problems 
in such a complex form that it is not possible to analyze 
the result in terms of direct plant response. Just as the 
agronomist's work is important, however, in securing such 
general results, that of the physiologist is important in the 
attempt to simplify conditions, to analyze factors, and 
ultimately to determine the nature of the plant response. 

72. The ash content of plants. — It has been noted 
that water constitutes ordinarily about four fifths of the 
weight of herbaceous plants. The remainder is solid 
matter. When the latter is burned in an open fire, the 
organic products are volatilized, and most of the mineral 
136 



Mineral Nutrients 



137 



constituents remain in the ashes (technically the ash). 
Water, total solid matter, and ash are therefore readily 
determined by simple methods. 

The total ash seldom amounts to more than 2 or 3 per 
cent of the green- weight, and any single mineral element 
of this ash constitutes, as a rule, merely a fraction of 1 per 
cent of the weight of the plant ; yet every essential mineral 
element is quite as important as any other factor in plant 
production. The percentages of ash in some familiar 
plants and plant products are given in the following 
table : l — 



Product 


. Total Solids 


Ash, Per Cent of 
Total Product 


Ash, Per Cent in 
Water-free 
Substance 


Corn, green fodder . 


20.67 


1.16 


5.6 


Corn, ripe grain . . 


89.44 


1.53 


1.7 


Sorghum, green fodder 


20.60 


1.09 


5.3 


Wheat, ripe grain ' . . 


89.48 


1.87 


2.0 


Timothy, green hay . 


38.42 


2.10 


5.4 


Red clover, green hay . 


29.21 


2.10 


7.2 


Red clover, cured hay . 


84.76 


6.15 


7.3 


Alfalfa, green hay . . 


28.25 


2.66 


9.4 


Red beets . . . 




11.53 


1.04 


9.1 


Sugar beets 






13.50 


.88 


6.5 


Turnips 






9.54 


.80 


8.4 


Cucumbers 






4.01 


.46 


11.5 


Cabbages . 






9.48 


1.40 


14.8 


Lettuce 






4.13 


1.49 


19.0 


Apples, R.I.G 






17.50 


.80 


2.1 


Strawberries, fruit 




9.16 


.60 


6.5 



1 Many of these are the average results of Jenkins and Winton (Com- 
pilation of Analysis of American Feeding-stuffs, Off. Exp. Stas. U. S. 
Dept. Agl., Bui. ii : 156 pp., 1902). 



138 Plant Physiology 

Knowledge of the ash content is of interest physiologi- 
cally when related to plant behavior or work. 

73. Composition of the ash. — A detailed analysis of 
the constituents of the ash indicates that through absorp- 
tion the plant obtains, as a rule, more or less of all of the 
soluble mineral elements of the soil. Whatever occurs 
in the soil solution is apt to be found in the plant to at 
least a slight extent, although the plasmatic membranes 
of the root-hairs show a certain definite selective absorp- 
tion, as already indicated. Commonly ejeven elements 
are found in the ash, as follows : phosphorus, potassium, 
calcium, magnesium, sulfur, iron, sodium, chlorine, silicon, 
manganese, and aluminium ; and, generally speaking, 
the soil is the only source of these elements. (Nitrogen, 
likewise derived from the same source, is, of course, a part 
of the volatile product.) 

Chemical analysis cannot determine with any degree of 
exactness what the plant actually requires from the soil; 
but it is important because it gives a general indication of 
the relation of plant to soil solution, it sheds some light 
upon the general problem of nutrition, and it makes pos- 
sible an exact computation of the amounts of mineral 
nutrients which various crops remove from the soil. The 
table on the opposite page compiled by Kedzie shows the 
percentage composition of the ash of familiar crops. 

From these data it is obvious that there are certain 
general relations worthy of recollection, such as these : 
the seed is relatively rich in phosphorus and magnesium, 
and usually deficient in calcium ; stems and leaves may 
contain much calcium, and often a high per cent of silicon ; 
while the fleshy roots here noted show the highest potas- 



Mineral Nutrients 



139 



Composition of 100 Parts of the Pure Ash 





K 2 


Na 2 


CaO 


MgO 


Fe 2 3 P 2 O s 


S0 3 


Si0 2 


CI 


Seeds 




















Wheat .... 


30.24 


.65 


3.50 


13.21 


.60 


47.92 





.73 


- — 


Corn 


29.8 


1.10 


2.17 


15.52 


.76 


45.61 


.78 


2.10 


.91 


Flax 


26.67 


2.22 


9.61 


15.86 


1.11 


42.48 





.88 





Clover .... 


35.35 


.95 


6.40 12.90 


1.70 


37.93 


2.40 


1.30 


1.23 


Beans 


41.48 


1.10 


4.9!) 


7.15 


.46 


38.86 


3.40 


.65 


1.80 


Fodders 


















Clover .... 


27.25 


.80 


29.20 


s.32 .57 


10.66 - 


6.18 





Timothy .... 


34.69 


1.83 


8.05 


3.24 .83 


11.80 2.80 32.17 5.20 


Corn . . . ... 


27.18 


.85 


5.70 


11.42 .85 


9.14 - 


40.18 - 


Straws 




















Flax 


34.07 


4.37 


24.81 


15.04 


3.67 


6.24 





6.70 





Buckwheat . . . 


46.60 


2.20 


18.40 


3.60 





11.19 


— 


5.50 





Wheat .... 


13.65 


1.38 


5.76 


2.46 


.61 


4.S1 





67.50 





Roots 




















Potatoes .... 


60.00 


2.96 


2.64 


4.93 


1.10 


16.86 


6.50 


2.10 


3.40 


Sugar-beets . . . 


53.10 


8.92 


6.10 


7.86 


1.14 


12.20 


4.20 


2.28 


4.80 


Turnips .... 


45.40 


9.84 


10.60 


3.69 


.81 


12.71 





1.80 


5.00 



sium content. Different parts of the same plant may 
exhibit great diversity in ash content, indicating an im- 
portant selective absorption between cells and organs. 

74. Effects of conditions upon ash content. — For any 
particular plant or plant product produced under diverse 
conditions the ash content is subject to considerable varia- 
tion ; and this variation, while most marked with res] feet 
to total ash content, extends also to the ratio of the dif- 
ferent elements of which the ash is composed. Official 
analyses of the sugar-beet show an ash content, calculated 
to dry- weight, of from 3.2 to 14.6 per cent. There may be 
a difference with varieties of any plant, but even in the 



140 Plant Physiology 

same variety a marked difference will result when plants 
grown in Michigan, for example, are compared with those 
grown on alkali land under irrigation in Colorado. It is 
to be expected, therefore, that there will occur considerable 
variation in the ash under different conditions of soil 
water, fertilization, temperature, and light, or under any 
conditions affecting transpiration and growth. 

75. Ash content at different ages. — It is of interest to 
note that at different stages of growth the rate of absorp- 
tion of mineral nutrients and nitrogen bears no constant 
relation to body weight. Arendt, 1 Bretschneider, 2 and 
others have shown that in general ash and nitrogen are 
present in the young plant in relatively greater quantities 
than in later stages of growth ; while starch accumulates 
relatively more rapidly in the maturing plant. Each of 
the observers referred to employed oats, and they divided 
the growing period into five intervals practically as fol- 
lows : (1) as three to five leaves are unfolded, (2) some- 
what previous to full heading, (3) plants in full blossom, 
(4) beginning of ripening period, and (o) complete matur- 
ity. In these experiments the roots were not taken 
into consideration. The table on the opposite page 
from Bretschneider shows the absorption of total ash 
and of nitrogen during different stages. With respect to 
the absorption of individual constituents, phosphoric acid 
is obtained in relatively greater quantity during heading, 
while potash is more rapidly absorbed during the early 
stages, according to Arendt. 

1 Arendt, " Waehsthumverhaltnisse der Haferpflanze." Jour. f. prakt. 
Chem., 76: 193, 860. 

2 Bretschneider, "Das Wachsthum der Haferpflanze." Leipzig, 1859. 



Mineral Nutrients 



141 



Period 


Ash 


Nitrogen 


i 


8.57 


3.59 


3} 


5.96 


2.79 


4 


5.33 


2.78 


5 


5.40 


2.43 



76. Translocation of mineral substances. — It is well 
known that as a plant begins to develop fruit or seed there 
is generally a movement of certain elements or substances 
to these parts. They may become the storage organs of 
carbohydrates, proteins, and other organic compounds, 
but there is also a selective absorption of mineral constit- 
uents. Throughout the period of fruit formation phos- 
phoric acid migrates toward the fruiting organs from leaves 
and stems ; and magnesium is invariably translocated, espe- 
cially from the lower leaves and stem to the younger organs 
of the upper portion, or to the fruit. Potassium frequently 
reaches a maximum in the fruiting organs at the time of 
blossoming, and subsequently may be slightly replaced by 

Phosphoric Acid Context at Various Stages of Growth 



Part op Plant 


Periods of Growth 




1 


2 


3 


4 


5 


Three lower joints, stem . 
Two middle joints, stem . 
Upper joint, stem . . . 
Three lower leaves . . . 
Two upper leaves . . . 
Ear 


.47 

1.05 
1.75 


.20 
.39 
.66 
.70 
1.67 
2.36 


.21 

1.14 
1.73 
.69 
1.18 
5.36 


.20 
.46 
.31 
.51 
.74 
10.67 


.19 
.18 
.36 
.35 
.59 
12.52 



142 Plant Physiology 

magnesium. Such substances as lime, silicon, and chlorine 
do not seem to move appreciably. According to Arendt 
1000 oat plants contained in the various periods of growth 
the quantities of phosphoric acid given in the preceding 
table, expressed in grams. 

77. Water cultures. — From a study of the nutrient 
requirements of plants in soils, or even in sand cultures, it 
is not possible to arrive at a definite conclusion respecting 
the elements needed by plants through the soil solution. 
For this purpose water cultures are required, and such 
cultures have been employed for more than half a century 
in the study of plant nutrition and other physiological 
relations. Relatively simple experiments afford the chief 
fundamental facts. Many plants lend themselves to 
water-culture experiments ; in fact all cereals, peas, beans, 
buckwheat, and many other crop plants may be employed, 
in spite of the unusual conditions to which the roots are 
subjected. 

The seed represents a considerable accumulation of 
necessary mineral nutrients as well as organic foodstuffs, 
and if so supported that the roots may grow in a vessel of 
distilled water, this supply of nutrients alone may support 
a strong growth for one or two weeks. If peas or beans are 
employed and the cotyledons are cut off as soon as the plu- 
mule is well developed, the growth in distilled water will be 
very slight. Ordinary well water, or the seepage water from 
a tile drain, used as a culture medium, and frequently re- 
newed, affords a vigorous growth. 

A water culture containing as soluble salts the elements 
nitrogen, phosphorus, potassium, calcium, magnesium, 
sulfur, and iron will afford more or less perfect growth. 

C Hopk.K G.& 



Mineral Nutrients 



143 




144 Plant Physiology 

These seven elements, in addition to hydrogen, oxygen, 
and carbon (this last supplied by the carbon dioxid of the 
air, see Chapter IX), are those indispensable for green 
plants generally ; and the absence of any one of the seven 
in the nutrient solution will eventually result in the ces- 
sation of growth. 

In the preparation of the cultures it is convenient to 
employ as culture vessels ordinary glass tumblers (Fig. 44) 
covered with black paraffined paper, preferably doubled; 
a shell of black paper is also fitted over the remainder of 
the tumbler, and wire guards to assist in supporting the 
plants as they grow are attached with rubber bands. 
Canada field peas (Pisum arvense) give a quick growth, 
and are satisfactory for this work. They have the dis- 
advantage of being unusually sensitive to a lack of calcium, 
as discussed later. Wheat or oats may be used, and these 
do well, especially when the solution is often renewed. 
In general, the cereals in solution culture respond quickly 
at the outset to potassium and to nitrogen, and, relatively 
speaking, there is often a deficiency of these elements in 
the seed. 

Solution cultures in vessels of the size above noted are 
important merely for those observations extending over 
comparatively short periods. Large vessels of the nature 
of battery jars, permitting the use of several liters of the 
solutions, are required when it is desirable to bring the 
plants to an advanced state of growth, or to maturity. 

78. Nutrient solutions and water cultures. — The nu- 
trient solution may be variously constituted. It must 
contain the elements previously mentioned, and it may be 
well to include also sodium and chlorine. It is probable 



Mineral Nutrients 145 

that there is no one ideal nutrient solution, since plants 
vary considerably in their requirements. The solutions 
given below have been much employed, and they are 
among those that are generally satisfactory : — 

1. Pfeffer's Solution . 

Calcium nitrate . ■ 4 grams 

Potassium nitrate 1 gram 

Magnesium sulfate 1 gram 

Potassium dihydrogen phosphate 1 gram 

Potassium chloride 5 gram 

Ferric chloride trace 

Distilled water 3 to 7 liters 

2. Crone's Solution 

Potassium nitrate 1.00 grams 

Iron phosphate 50 grams 

Calcium sulfate 25 grams 

Magnesium sulfate 25 grams 

Distilled water 2.0 liters 



The first solution has been more commonly employed. 
For different plants it is particularly important to change 
the ratio of calcium to magnesium. This is conveniently 
done by reducing the amount of calcium nitrate and adding 
to the potassium nitrate. The Crone solution is reported 
satisfactory for cereals ; but it is more difficult to handle 
on account of the relatively insoluble iron phosphate. 

Water cultures for most seed -plants are preferably 
slightly acid at the outset, especially where the solution is 
constituted as in number one. This solution becomes 
alkaline in time. In the preparation of these solutions for 



146 



Plant Physiology 



careful work only the purest distilled water should be em- 
ployed, and in no case should water from a copper still 
be considered acceptable for water cultures. Water 
double distilled from hard glass is preferably employed in 
accurate work. A method in use at the Bureau of Soils 
and elsewhere for the treatment of distilled water, such as 




Fig. 45. Tobacco in continuous culture : Plat 9, "complete" fertilizer; 
Plat 10, unfertilized. [Photograph from the Ohio Agl. Exp. Sta.] 

might be obtained from a tin-worm still, for instance, con- 
sists in shaking up the water with carbon black, or with 
iron hydrate to free it of any injurious substances. 

79. Strength of the nutrient solution. — In the PfefTer 
and Crone solutions given above, the concentration is 
about .1 per cent, or 1 part in 1000 parts of water. 
This concentration is for many plants sufficiently favorable 



Mineral Nutrients 147 

in solution cultures. According to Nobbe, nutrient solu- 
tions half this strength, on the one hand, or three times as 
strong, on the other, failed to give the best results with a 
majority of the plants tested. The higher concentration, 
however, is far too weak to produce any immediately 
recognizable osmotic disturbance. The plasmolyzing con- 
centration of KN0 3 , for example, would be for many plants 
10 to 15 parts per thousand. Nevertheless, from the above 
facts, it is evident that aside from all financial' considera- 
tions in the application of fertilizers there is a definite 
physiological limit to the application of soluble commercial 
manures. 

Neglecting for the moment the effect of the soil upon 
solubility, an extreme case may be taken : assume that 
500 pounds per acre of fertilizer are applied to a loamy 
soil, and that all of the fertilizer goes into solution. If 
the water-holding capacity of the soil is 40 per cent and 
the actual water-content, say, 15 per cent, there would be 
in the upper 7 inches of soil about 315,000 pounds of water, 
and the concentration of the KN0 3 alone would be 1J parts 
per thousand. This calculation is, of course, far from 
what would actually occur, for the soil is a strong absorp- 
tive matrix, and by no means all of a soluble nutrient added 
would be effective in the soil solution. Moreover, in most 
cases, a relatively small quantity of the fertilizers added 
remains in soluble form. 

Commercial fertilizers applied in the drill or in contact 
with the seed may readily be present in sufficient quantity 
to be injurious to the germinating seedling. Claudel and 
Crochet elle 1 find that solutions of 1 to 1000 of ammonium 

1 Annales agron., 22 : 131-142, 1896. 



148 Plant Physiology 

sulfate, sodium nitrate, and some other salts are injurious 
when applied to seed in pure sand. Newman ! concluded 
that on sandy soil 400 pounds of sodium nitrate is unfavor- 
able to the germination of peas. Hicks, 2 reporting upon 
the germination of seeds as affected by diverse fertilizers, 
states that, " commercial fertilizers should not be brought 
into direct contact with germinating seed." 

80. The forms of the nutrient compounds. — Since the 
mineral nutrients (including nitrogen) are available to the 
plant usually only through the soil solution, it is a general 
rule that any soluble inorganic salts which are not toxic, 
or poisonous, may supply the nutrient or nutrients needed. 
Nitrogen, for instance, in water cultures may be supplied 
in the form of any of the nitrates. In the field it could not 
be supplied either as calcium or magnesium nitrate, on 
account of the greater expensiveness of these compounds. 
It may be supplied as potassium nitrate, saltpetre; but 
more extensively as sodium nitrate, or Chilian saltpetre, 
a common fertilizer. Again, nitrogen is to a certain extent 
supplied as ammonia compounds, the compound of prac- 
tical importance being ammonium sulfate. Ammonium 
compounds are further readily diffused through the soil, 
and if not used directly, they are, by microorganisms, 
easily converted into nitrates ; hence they may be con- 
sidered, in general, as readily available forms of this the 
most expensive of the nutrient elements. In this connec- 
tion it is important to note that it is only after decompo- 

1 Arkansas Agl. Exp. Sta., Bui. 34 : 99-124. 

2 Hicks, G. H., " The Germination of Seed as Affected by Certain 
Chemical Fertilizers." Div. Bot. U. S. Dept. Agl., Bui. 24 : 15 pp., 2 ph., 
1900. 



Mineral Nutrients 



149 



sition and conversion into ammonia and nitrates that the 
numerous important organic nitrogen fertilizers, such as 
stable and green manures, dried blood, tankage, and the 
like, are to any practical extent valuable for plants. De- 
composition and nitrification processes, however, will be 
discussed later. 




Fig. 46. Tobacco experiments : Plat 1, no fertilizer ; Plat 2, acid phos- 
phate. [Photograph from the Ohio Agl. Exp. Sta.] 



The soluble phosphates of the various bases are all imme- 
diately available and may be used in water cultures ; but 
phosphates are frequently applied to the soil in some 
insoluble form, such as bone-meal or phosphatic rock, 
which become gradually available by chemical changes in 



150 Plant Physiology 

the soil, and by root action. The common phosphatic 
fertilizers are the four phosphates of lime, and the only one 
of these which is soluble is the saturated, or superphos- 
phate [Ca(H 2 P0 4 ) 2 4- H 2 0], although the reverted or dical- 
cic phosphate is also readily available. 

Many soluble forms of potash might be used, but the 
important commercial forms are the sulfate, chloride 
(muriate), and carbonate. In quantity the chloride is 
injurious to some crops. The chief sources besides ashes 
are now the crude products of the German potassic 
mines. 

81. Plant nutrients in rock. — It is more particularly 
the province of instruction in soils and economic geology 
to consider the origin of the plant nutrients of the soil. 
The geological history is, of course, of no physiological 
significance ; it is information ; so that it is here sufficient, 
by way of reference, to note some few of the more impor- 
tant facts. 

The rocks of the earth's crust from the oldest to the most 
recent, from the hardest to the softest, whatever may have 
been their origin, are made up of a variety of minerals, 
some of the chemical constituents of which are the ele- 
ments previously noted as necessary in the growth of 
plants. Even the hardest granites, basalts, and lavas 
contain, in general, a small percentage of potash, soda, 
lime, magnesia, and iron. A single form of rock, such as 
one of the red granites, may be deficient in magnesia; 
another, like a red, soil-forming basalt, may lack in potash ; 
whilst a limestone may contain no iron. The plant nu- 
trients form commonly a minor portion of the bulk of the 
rock, the balance consisting often of silica and alumina. 



Mineral Nutrients 



151 



The following table shows the distribution of the sub- 
stances mentioned in a few types of rocks : 1 — 



Gra- 
nitic 



Basal- 
tic 



Lava 
(Vesu- 
vius) 



Lime- 
stone 
(Vir- 
ginia) 



Lime- 
stone 
(Dolo- 

MlTir) 



Silica (SiO a ) . . . 
Alumina (A1 2 3 ) 
Ferric oxid (Fe 2 3 ) 
Ferrous oxid (FeO) 
Lime (CaO) . . . 
Magnesia (MgO) . 
Soda (Na 2 0) . . 
Potash (K 2 0) . . 
Phosphoric acid (P 2 5 ; 
Carbonic acid (C0 2 ) 
Ignition and loss . 



69.80 

14.45 

2.62 

1.94 

1.84 

.49 

3.91 

3.96 

.10 

.89 



53.13 
13.74 
1.08 , 
9.10] 

9.47 
8.58 
2.30 
1.03 
.40 

1.17 



48.12 
17.16 

10.82 

9.84 
3.99 
2.77 
7.24 



4.13 

4.19 

4.33 

2.35 

44.79 

.30 

.35 

.16 

3.04 

34.10 



53.4 
44.2 



Practically, no soil is made up of mineral constituents 
in the same proportion as they occur in the original rock. 
There are losses and gains of plant nutrients with respect 
to any one type of rock, but residual soils, referable to the 
decomposition of a particular rock, approach the rock 
more nearly. In general, however, it is clear that, since 
soils are formed by the grinding down and decomposition 
of rocks, they may contain all the minerals- of the earth's 
crust. A soil is ordinarily of complex origin, and aside 
from this, the chief qualities which make it a favorable 
environment for the plant as contrasted with broken rock 

1 For a comprehensive treatment of the composition of original rock 
and residual soils derived therefrom the student should consult the 
following: Lyon and Fippin, "Soils," pp. 1-68; Bailey, "Cyclopedia of 
American Agriculture," i (Chapter X) : pp. 323-371. 



152 Plant Physiology 

or coarse sand may be chiefly three: (1) comminution 
and greater water-holding capacity, previously discussed; 

(2) the addition of accumulated organic matter, and 

(3) the presence of a variety of microorganisms, gradually 
transforming the organic matter. The fine state of divi- 
sion of the soil particles also permits great freedom to 
the further weathering influences of water and other 
factors concerned in the rock disintegration which is con- 
stantly in progress. 

82. Soil fertility. — Fertile soils will generally contain 
an abundance of the soil nutrients, sufficient to produce 
crops for many successive years. This does not necessarily 
imply that the nutrients are available in proper ratio. 
Intelligent growers, moreover, consider not merely the pres- 
ent production of crops, but also the maintenance of high 
fertility in the case of fertile soils, and the development 
of fertility in unproductive soils. It is necessary, then, 
to have in mind the supply and the source of supply of the 
important elements and their relative abundance. 

Sulfur and iron may be dismissed from further considera- 
tion, since they are naturally abundant in soils, and are 
used by plants in such limited quantities that a dearth 
of these nutrients is not common. As would be expected 
these two elements are only incidentally constituents of 
commercial fertilizers. Magnesium is also ordinarily 
present in sufficient quantities, and it may be present in 
such excess as to be harmful, as noted later. The plant 
producer is now certain that more attention must be paid 
to lime, and especially to the relative abundance of lime 
and magnesia. Furthermore, when liming is required 
every few years, it is a good custom to determine for any 



Mineral Nutrients 



153 



soil the value of using, about once in twelve or fifteen years, 
a lime with high magnesia content. Finally, of all the 
important elements furnished by the soil, nitrogen, phos- 
phorus, and potassium are less abundant, relatively more 
in demand by the growing crop, and accordingly to be 
conserved and consistently restored. 

83. Nutrients removed by farm crops. — In order to 
appreciate properly the relation of cropping to the fertility 
of the land it is necessary first to note the amounts of the 
more important soil nutrients — nitrogen, phosphorus, 
and potassium — which may be removed by various 
crops annually. The table below is adapted from data 
given by Hopkins : l — 



Amount in Pounds removed 



Nitrogen 



Phos- 
phoric 
Acid 



Value of 
Nutri- 
ents per 
Potash A - 



Alfalfa hay . . 
Clover hay . . 
Timothy hay 
Potatoes, tubers 
Sugar-beets, roots 
Corn, grain . 
Corn, stover 

Corn crop 
Wheat, grain 
Wheat, straw 

Wheat crop 
Apples . . 



only 



5 tons 

3 tons 

2 tons 

200 bu. 

15 tons 

60 bu. 

1.8 tons 



30 bu. 
1.5 tons 



250.0 
120.0 
48.0 
42.0 
80.0 
60.0 
28.8 
88.8 
42.6 
15.0 
57.6 
31.3 



22.5 

15.0 

6.0 

8.7 

14.4 

10.2 

3J> 

13.8 

7.2 

2A 

9.6 

3.3 



120.0 
90.0 
47.7 
60.0 
125.6 
11.4 
31.2 
42.6 
7.8 
21.0 
28.8 
38.0 



47.40 
25.20 
10.76 
10.94 
21.26 



11.52 

7.38 



1 Hopkins, C. G., "The Fertility in Illinois Soils.' 
123 : p. 189. 



111. Agl. Exp., Bui. 



154 



Plant Physiology 



In the preceding table it is shown significantly that 
the amount of nitrogen taken up by the leguminous crops 
reaches a figure averaging far above that of the others. 
As indicated later, much of this nitrogen is derived from 




Fig. 47. Fertilizer experiments with cereals 

effect of a nitrate. 



the air through the remarkable activity of the bacteria of 
the root tubercles ; and in reality it often represents, even 
with the harvesting of the crop, a soil gain. It would 
represent a large soil gain if the crop were returned to the 



Mineral Nutrients 



155 



land. In general, for the crops included, the losses of 
nitrogen and potash are fairly comparable, while the loss 
of phosphorus is only about one fourth as great as either 
of the other constituents. Frequently, however, much 
of the potash-containing products, such as straw and 
stover, are returned to the land. 

84. Nutrients removed by fruit crops. — The following 
table indicates the amounts of nitrogen, phosphoric acid, 
potash, and lime removed by various fruits, the quantities 
being determined for the fresh fruit per thousand pounds, 
assuming that leaves, wood, etc., of the trees will be 
eventually returned to the soil : — 

Quantities of Soil Ingredients withdrawn by Various Fruits 1 



Fresh Fruit 
1000 Pounds 



Almonds 2 . 

Apples . . 

Apricots . . 

Bananas . . 

Cherries . . 

Chestnuts 2 . 

Figs . . . 

Grapes . . 

Olives . . . 

Oranges . . 
Prunes, French 
Walnuts 2 



Amounts of Nutrients Removed, in Pounds 



I Phos- 

Total Ash Nitrogen phoric 

J Acid 



17.29 
2.64 
5.08 

10.78 
4.82 
9.52 
7.81 
5.00 

13.50 
4.32 
4.86 

12.98 



7.01 


2.04 


1.05 


.33 


1.94 


.66 


.97 


.17 


2.29 


.72 


6.40 


1.58 


2.38 


.86 


1.26 


.11 


5.60 


1.25 


1.83 


.53 


1.82 


.68 


5.41 


1.47 



9.95 

1.40 
3.01 
6.80 
2.77 
3.67 
4.69 
2.55 
9.11 
2.11 
3.10 
8.18 



1.04 
.11 
.16 
.10 
.20 

1.20 
.85 
.25 

2.43 
.97 
.22 

1.55 



1 Data taken from Wickson, E. J., 

2 Including hulls. 



'California Fruits," p. 157, 1900. 



156 



Plant Physiology 



It is also further interesting to note the requirements 
per tree and also per acre in the case of certain fruits, as 
shown in the following tables reported by the New York 
Experiment Station : — 

Important Nutrients used during a Growing Season by 
Mature Fruit Trees 1 





Amount Removed per Tree, in Pounds 


Fruit 


Nitrogen 


Phos- 
phoric 
Acid 


Potash 


Lime 


Magnesia 


Apple ....... 

Peach 

Pear 

Plum 

Quince 


1.47 
.62 
.25 
.25 
.19 


.39 
.15 
.06 
.07 
.06 


1.57 
.60 
.27 
.32 
.24 


1.62 
.95 
.32 
.34 
.27 


.66 
.29 
.09 
.11 
.08 



Nutrients Used per Acre by Different Fruit Trees 2 







Amount removed per 


\., in Pounds 




No. 
Trees 








Variety 




Phos- 










per A. 


Nitrogen 


phoric 
Acid 


Potash 


Lime 


Magnesia 


Apple .... 


35 


51.5 


14.0 


55 


57.0 


23 


Peach .... 


120 


74.5 


1S.0 


72 


114.0 


35 


Pear .... 


120 


29.5 


7.0 


33 


38.0 


11 


Plum .... 


120 


29.5 


8.5 


38 


41.0 


13 


Quince . . . 


240 


45.5 


15.5 


57 


65.5 


19 



1 N. Y. Agl. Exp. Sta., Bui. 265 : p. 366. 

2 Ibid., p. 369. 



Mineral Nutrients 



157 



Amounts of Nutrients removed per Acre by the Fruit 1 Alone 



Part op Tree 


Variety of 
Fruit Tree 


Nitrogen 


Phos- 
phoric 
Acid 

(p 2 o 6 ) 


Potash 
(K 2 0) 


Lime 
(CaO) 


Mag- 
nesia 
(MgO) 






Pounds 


Pounds 


Pounds 


Pounds 


Pounds 


Fruit . . . 


Apple 


20.0 


8.5 


45.0 


3.9 


6.4 


Fruit . . . 


Peach 


17.5 


8.6 


36.0 


2.2 


4.1 


Fruit . . . 


Pear 


9.0 


3.2 


20.2 


2.2 


2.6 


Fruit . . . 


Plum 


13.3 


4.7 


18.5 


4.4 


3.0 


Fruit . . . 


Quince 


20.0 


10.0 


44.4 


3.4 


6.0 



Assuming that the leaves, dead twigs, etc., are annually 
returned to the soil, fruits are ordinarily less exhausting 
than field crops. In this connection it is entirely imma- 
terial that a bushel of oats of the same variety, or a barrel 
of Baldwins, will not alwaj^s contain the same amounts of 
nitrogen, phosphoric acid, and potash. Other analyses, 
therefore, will not accord in detail with those given. The 
fact that these crops are not exhausting is important in con- 
templating the maintenance of fertility in intensive fruit 
production. It would seem that in fruit production it 
may easily be possible to realize a permanent system of 
agriculture. 

85. Amount of nutrients in soils. — Fertility is a matter 
so complex — dependent upon such a variety of factors 
— that a chemical analysis is important in two respects, 
chiefly: (1) to indicate the total amounts of plant food, 
for the time available or unavailable, and (2) to point out 
unbalanced conditions, or to suggest lines of treatment. 
Ultimately, experiments with the plant are invariably 

1 Ibid., p. 370. 



158 Plant Physiology 

required in order to determine what is, for any soil, the 
most effective fertility. 

Many analyses of tillable soils have been made through- 
out the United States, and it is shown that the storage of 
nutrient elements therein is most diverse. Calculated to 
pounds per acre in the upper seven inches of soil many 
complete analyses afford extremes as shown in the follow- 
ing tabular summary : — 



Nutrient 


Amount per A. 


Phosphoric acid 

Potash 


Pounds 

500 to 10,000 

3000 to 100,000 

2000 to 200,000 


Magnesia 


1500 to 150,000 



When the distribution of the roots and the availability 
or lack of availability of the compounds are taken into 
consideration, it is evident that with respect to the minima 
one may speak of exhaustion or lack of nutrients, but with 
respect to the maxima there may be sufficient, conserva- 
tively used, for generations. 

A very thorough study is being made of the soils of 
Illinois, and the table on the opposite page gives the 
average amount of plant food for a variety of soil types. 

These represent, except in the last two cases, the total 
plant food in 2,000,000 pounds of dry surface soil, this 
weight being approximately that of 7 inches of ordinary 
soil. In the case of sand, which is heavier, about 
2,500,000 pounds are concerned in the same depth, and 
in the case of the light peat only about 1,000,000 pounds. 



Mineral Nutrients 



159 



Gray silt loam 
Brown silt loam . 
Black clay loam . 
Yellow silt loam . 
Brown sandy loam 
Brown bottom loam 
Sandy soil . . . 
Deep peat . 



Total 
Nitrogen 



2,880 
5,035 
7,228 
2,016 
3,070 
4,720 
1,440 
34,880 



Total 
Phos- 
phorus 



840 
1230 
1755 

884 

850 
1620 

820 
1960 



Total 
Potas- 
sium 

24,940 
35,792 
33,510 
33,901 
26,700 
39,970 
30,880 
2,930 



The vast array of facts which have been developed with 
respect to the amount of nutrients in soils is, after all, of 
somewhat limited application. This is due in large part 
to fundamental difficulties in obtaining a satisfactory 
basis for a computation of effectiveness. If, for ex- 
ample, the total quantities of the nutrients contained 
in the first few inches of soil are made a basis, then, to 
modify the calculations of the amounts of the nutrients, 
there are such conflicting factors as the following : — 

(1) Roots are not commonly limited to the first 7 or 
10 inches of soil (Fig. 9). 

(2) To a certain extent there is in the soil a movement 
of the soluble nutrients from higher to lower levels and 
also the reverse. There is further a variable loss from 
leaching. 

(3) The roots do not actually come in contact with al] 
of the soil, and the special solvent action (discussed later) 
is greatest in the immediate vicinity of these structures. 

(4) No absorbing organs are able completely to " ex- 
haust " or remove all the nutrients from any soil, and the 
amounts readily removable depend upon complex chemi- 
cal and physical factors. 



160 Plant Physiology 

86. Availability of the nutrients. — Plant nutrients 
exist in the soil in conditions most diverse with respect to 
availability, and chemical analysis does not satisfactorily 
distinguish between availability and nonavailability. 
Potassium, for example, may be present in conjunction 
with aluminium silicate, or it may be present in far more 
soluble form ; but there are at present very few data con- 
cerning the nature of these compounds. If any element 
is present in markedly unavailable form, that element 
will be needed as a fertilizer, especially to hasten the early 
stages of growth. 

Fertilizers are generally applied, not only to keep up 
fertility, but to increase availability. In the latter case, 
therefore, from the immediate standpoint of the plant, 
fertilizers are supplied either (1) as directly available 
nutrients; (2) as substances, effecting readjustments in 
the soil, so that needed elements become more available 
to the plant; or (3) in order to counteract the effects due 
to some unbalanced condition of the nutrients (later dis- 
cussed at length), injurious acidity, alkalinity, and the 
like. 

87. The solvent action of roots. — It is well known 
that roots and root-hairs are able to render available a 
certain amount of nutrient materials. There is a solvent 
action of the roots. The case almost universally cited is 
the corrosion of marble (limestone) by roots. The nature 
of this solvent action has been much studied and discussed. 
It is certain that the excretion of aqueous C0 2 is sufficient 
to account for much, and probably for nearly all of this 
action. 

Kunze and others seem to have convincingly demon- 



Mineral Nutrients 161 

strated that there is no excretion of a mineral acid, and that 
any organic acids present are beyond the sensitiveness of 
litmus. Nevertheless, some investigators have found other 
acids present under certain conditions. These conditions 
are mainly poor oxygen supply. Stoklasa and Ernst, 
for example, have identified traces of acetic acid and 
formic acid with poor oxygenation of the roots of corn and 
barley. Under such circumstances these may be regarded 
as evidence of unfavorable surroundings, and not as excre- 
tions beneficial to the plant. Under similar circumstances 
oxalic acid was identified in the case of the sugar-beet 
and of the hyacinth. From the preceding it seems safe 
to assume that in the case of cultivated plants normal 
solvent action is due to C0 2 (an excrete product produced 
by every living cell ; cf . respiration) . 

It should be observed, however, that recent studies 
by Schreiner and Reed call special attention to the 
oxidizing action of roots. This seems to be brought 
about by a peroxidase, and the process may be practically 
important, since many cultural practices are designed 
to promote oxidation. 

88. C0 2 excretion and the availability of phosphorus. 
— Stoklasa and Ernst have further given some data indi- 
cating that the relative rate of excretion of C0 2 by the 
roots per gram of dry weight of substance is directly im- 
portant in determining the capacity of a plant to get phos- 
phoric acid from the more insoluble substrata. The follow- 
ing table exhibits side by side the excretion of C0 2 , as 
shown by water cultures, and the absorption of phosphoric 
acid when the same kinds of plants are grown in gneiss 
and basalt : — 



162 



Plant Physiology 



Plants grown 90 


Days at 20° C. 


Plants grown 77 Days 




C0 2 excreted per 
1 gr. Substance. 
Mg. per 24 hr. 


Substratum 


P 2 5 , Per Cent in 
Dry Substance 


Barley .... 
Wheat .... 

Rye 

Oats 


74.6 
89.6 
110.8 
118.9 


f Gneiss 
Basalt 
j Gneiss 
i Basalt 
j Gneiss 
j Basalt 
| Gneiss 
( Basalt 


.285 
.297 
.363 
.443 
.368 
.465 
.534 
.631 



89. Another view of soil fertility. — In the discussion 
of fertility thus far it is accepted that soils may be rela- 
tively deficient in nutrients, that removal of nutrients 
by crops tends towards practical deficiency, and that the 
addition of fertilizers, although it may also affect avail- 
ability, or balance, is a considerable factor in maintaining 
fertility. Some investigators advocate another view. 
This is in part based upon a method of observation and 
experiment yielding results which seem to point to an 
unexpected uniformity in the constitution of the soil solu- 
tion from diverse types of soil. 

They conceive that the addition per acre of a few hun- 
dred pounds of fertilizers to one or two million pounds of 
earth (surface soil) is of no consequence in increasing 
available nutrients, and they would ascribe the admitted 
value of fertilizers to some more general effects upon the 
soil, all of which are not understood. This view would 
seem to demand that sodium chlorid would be, in general, 



Mineral Nutrients 163 

as valuable a fertilizer as potassium chlorid or sulfate. 
We must regard as one of many types of facts opposed to 
this view the vast amount of experimental work showing 
the direct value of particular nutrients, and more espe- 
cially, of particular nutrients at certain stages of the 
growth of the crop. The view is of undoubted value 
in suggesting lines of investigations. Associated with it, 
usually, is the idea of toxic excreta from plant roots, which 
is considered in another place. 

90. The paraffined wire basket in nutrition studies. — 
In determining through plant growth certain soil relations 
by a quick laboratory method it has been the custom to 
employ tumblers or other similar glazed vessels from which 
there could be no loss of the materials employed. These 
are not always satisfactory, since, if drying out proceeds, 
spaces are left between the soil and the vessels, and under 
unfavorable conditions, especially, it is in these spaces that 
the roots grow, thus giving no exact indication of the soil 
conditions. 

The paraffined basket method is well demonstrated by 
Figure 48, in which, from left to right, successive stages in 
the preparation of the culture are shown. As described 
by Schreiner, the basket is dipped top downward into hot 
paraffin several times until a rim is made. It is then filled 
with soil to the rim, and firmly packed near the gauze, 
the surplus protruding soil being brushed off. The basket 
is then dipped into the paraffin up to the rim several times. 
The paraffin penetrates into the soil pores or capillaries, 
and there is no line of cleavage, as with glazed vessels. 
The surface of the soil may be covered with paraffined 
paper, in which slits are made for placing the seedlings. 



164 



Plant Physiology 



The method has been much used in connection with 
Livingston's plan of using transpiration as a measure of 




Fig. 48. The paraffin-basket method. Upper illustration shows se- 
quence of stages in preparing cultures, and the lower a comparison of 
root growth in a basket (left) with a tumbler (right). [Photograph 
from the Bureau of Soils, U. S. Dept. Agl.] 

growth, but it has a much wider application, whatever 
the indicator may be, in the general study of the mineral 
nutrients of the soil, and many other soil conditions. 
This method is unnecessary where the conditions of soil 
moisture are constant, and with grades of coarse sand. 



Mineral Nutrients 165 

Again, such plants as corn and vigorous varieties of the 
sunflower are able to force the roots through the paraffin, 
especially in warm weather. 



LABORATORY WORK. — SUGGESTED EXPERIMENTS 

Solution cultures, essential nutrients. — Since the seed repre- 
sents a considerable accumulation of the necessary food-materials 
required by the growing plant, the absolute necessity of a particu- 
lar nutrient may not be readily demonstrated except by growing 
plants to maturity in relatively large vessels. The latter is com- 
monly impracticable, and in simple experiments it suffices to 
determine the comparative effects upon growth or green weight 
of a full nutrient solution, along with other solutions lacking 
each element in turn. While the method is open to criticism, 
the student will find much use for the experience in manipulation ; 
and after a study of balanced solutions, he may define his criti- 
cism. 

Materials needed : cheap tumblers covered and arranged as 
suggested (section 77 and figure 44), or wide-mouth bottles with 
flat corks notched to receive the seed ; black paper shells for 
darkening the cultures, and black paper circles or squares dipped 
in hot paraffin for tumbler covers ; chemicals required by the so- 
lution ; as many stock flasks, or bottles, as nutrients ; distilled 
water, graduates, rubber bands and labels ; and germinating 
seed. 

Uniform seedlings should be employed, and these should be 
grown on moist moss or sawdust, or upon a paraffined wire screen 
floated on water by corks, but sufficiently weighted to keep the 
seed moist. All vessels should be chemically clean (preferably 
by the acid-dichromate method), and only the purest chemicals 
and distilled water employed. 

Prepare a stock solution of each main constituent of the Pfeffer 
solution in the proportional quantity of water, thus for a 5000 cc. 
solution, as follows : — 



166 Plant Physiology 

Calcium nitrate, 4 grams in distilled water 1000 cc. 
Potassium nitrate. 1 gram in distilled water 1000 cc. 
Magnesium nitrate, 1 gram in distilled water 1000 cc. 
Potassium dihydrogen phosphate, 1 gram in distilled water 1000 

cc. 
Potassium chlorid, .5 gram in distilled water 1000 cc. 

Taking 50 cc. of each of the preceding, cultures of the full nu- 
trient solution lacking iron are prepared, the iron being added in 
every culture where desired by a few drops of a 2 per cent solu- 
tion of the salt indicated. 

In omitting the several elements separately, substitutions are 
made from solutions of other salts made up in the same propor- 
tion, but taking cognizance, in each case, of the smaller quantity 
desired, the following substitutions being recommended : — 

Less calcium, " use NaNO ;i . 

Less nitrogen, use CaCL and KC1, respectively. 

Less potassium, use NaNO^, NH 2 P0 4 , and XaCl, respectively. 

Less phosphorus, use KC1. 

Less magnesium, use Na^SO^. 

Less sulfur, use MgCl 2 - 

Set up duplicate cultures with full nutrient solution, with so- 
lutions lacking each element successively, with distilled water, 
with tap water, with Crone's solution. Also make for comparison 
cultures of the Pfeffer solution both five times the strength and 
one fifth the strength of that above used. Also set up two ad- 
ditional tumblers (employing the full nutrient solution) of peas 
to be employed in the last experiment. 

If it is possible to include tests with several plants, Canada 
field peas, oats or wheat, and buckwheat are important, for each 
manifests special requirements in the early stages of growth de- 
pending largely upon the composition of the seed. 

Use ten plants in each culture, keep in a fairly moist place (or 
invert a tumbler over each culture) for a day or two, then trans- 
fer to greenhouse, if possible. Replace, as needed, by pipette 



Mineral Nutrients 167 

the water lost by transpiration. If the cultures are continued 
longer than two weeks, renew the solutions. When necessary, 
support the plants by the wire standard and ring (Fig. 44). Close 
the experiment within four weeks, measuring tops, weighing roots 
and tops separately, taking notes on general appearance, tabulat- 
ing results, and representing graphically the green weight of tops 
and of the whole plant. 

Corrosion by roots. — Place the polished marble plates provided 
in the small germinating plats (cigar boxes 2 inches deep answer- 
ing well), cover with 2 inches of sand, sow seed of squash and bean, 
and maintain under conditions favorable for growth. When the 
seedlings have grown vigorously to a height of 5 or 6 inches, exam- 
ine the marble plates for etched tracings. 

Determination of acid excretion by roots. — When the two addi- 
tional tumblers employed in a preceding experiment with nutrient 
solution afford vigorous seedlings, set up the following experiment : 
Boil one liter of tap water in a flask, cool, and aerate, make slightly 
alkaline with potassium hydrate, and add a few drops of phenol- 
phthalein to give distinct pink color. With this solution fill four 
tumblers, two of which are to be covered with paraffined paper as 
controls ; to the other two transfer the covers and seedling peas 
above indicated. Place both sets under similar conditions, and 
after 24 hours note and compare the color in the two cases. If the 
pink color has disappeared, the solution has become acid. In 
that case pour the contents of one tumbler into an evaporating 
dish, and bring to a boil. If the color reappears promptly, it in- 
dicates carbonic acid. 



References 

Benecke, W. Die von der Cronesche Nahrsalzlosung. Zeitsch. 

Bot. 1 : 235-252. 
Breazeale, J. F. Effect of the Concentration of the Nutrient 

Solution upon Wheat Cultures. Science, N. S. 22 : 146- 

149, 1905. 
Cameron, F. K., and Bell, J. M. The Mineral Constituents of 



168 • Plant Physiology 

the Soil Solution. Bur. of Soils, U. S. Dept. Agl. Bui. 30 : 

70 pp., 1905. 
Fest, F. Ueber den zeitlichen Verlauf. der Nahrstoff-Auf- 

nahme [etc.]. Journ. f. Landw. 56 : 1-47, 1908. 
Hall, A. D. Fertilizers and Manures. 384 pp., 1909. 

The Book of the Rothamsted Experiments. 294 pp., 49 figs. 

Hopkins, C. G. Soil Fertility and Permanent Agriculture. 

653 pp., 1910. 
Hilgard, E. W. Soils. 593 pp., 89 figs., 1906. 
Lyon, T. L., and Fippin, E. O. Soils. 531 pp., 157 figs., 1909. 
Stoklasa, J., and Ernst, A. Beitrage zur Losung der Frage 

der chemischen Natur des Wurzelsekretes. Jahrb. f. wiss. 

Bot, 46 : 55-102, 1908. 
Voorhees, E. B. Fertilizers. 335 pp., 1907. 
Wheeler, H. J., and Adams, G. E. Concerning the Agricultural 

Value of Sodium Salts. R. I. Agl. Exp. Sta. Bui. 106 : 109- 

153, 1905. 
Whitney, M. A Study of Crop Yields and Soil Composition in 

Relation to Soil Productivity. Bur. of Soils, U. S. Dept. Agl. 

Bui. 57 : 127 pp., 24 figs., 1909. 
Wilfarth. Romer, u- Wimmer. Ueber die Nahrstoffaufnahme 

der Pflanzen in verschiedenen Zeiten ihres Wachsthums. 

Landw. Versuchsst. 63 : 1-70, 1905. 

Texts. Detmer, Johnson, Jost, Pfeffer, Sachs. 



CHAPTER VIII 

SPECIAL FUNCTIONS AND RELATIONS OF 
MINERAL NUTRIENTS 

THE ROLES OF MINERAL NUTRIENTS 

Plant physiological literature contains many references 
to the specific roles or effects of the various mineral nu- 
trients. Some of the observations and results are of par- 
ticular interest; but many of the suggestions are based 
upon such slight evidence as to require no consideration 
in this place. It is an interesting and important field of 
work, but explanations of many of the effects which have 
been noted are more easily formulated than proved, and 
a satisfactory interpretation of the results is proving most 
difficult. 

The method of inquiry involves, on the one hand, a 
study of the effects produced upon the plant or cell when 
an element is, as far as possible, eliminated ; or, on the 
other hand, observations upon the results of supplying the 
particular nutrient element under study when it has been 
deficient. These are practically the only methods which 
can be employed; but it must be admitted that the 
absence of any nutrient may lead to unbalanced conditions 
which may induce general pathological effects, so that the 
particular primary role may be obscured. An analogous 
169 



170 Plant Physiology 

criticism would be equally valid in many other lines of 
investigation. 

91. The nature of the special roles. — Certain soil ele- 
ments are needed in the building up of the permanent pro- 
teins of the living matter. Those which are known to 
enter invariably into the composition of albuminoidal or 
protein bodies are necessarily of first importance. Other 
essential mineral elements play only doubtful roles in pro- 
tein activities, yet they have evidently such important 
functions to perform in connection with the activities of 
the protoplasm and its products as to be indispensable. 

Practically, as expressed by Reed, we may say that in 
general essential elements appear to function in two ways : 
(a) as component parts of necessary cell structures and 
fluids ; and (6) as agents indirectly essential, by causing 
less understood physical or chemical reactions, — acting 
as carriers of other ions, as specific antidoting agents, or 
otherwise. 

The first group includes, among the elements now under 
discussion, nitrogen, phosphorus, and sulfur ; while 
potassium, calcium, magnesium, and iron fall apparently 
in the second group. If the chemical work of the future 
demonstrates fully the existence of the basic proteins, 
now postulated, as noted later, it would then only, perhaps, 
be safe to assume the incorporation of these elements into 
the protoplasm itself.' The latter elements (especially 
potassium) may be important in the osmotic work of the 
cell, requisite as carriers or accumulators of food atoms, 
as catalytic agents, etc. ; but with nitrogenous bodies 
like proteins they seem to form at most only temporary 
combinations. 



Special Functions and Relations 171 

92. The role of phosphorus. — Phosphorus is indispens- 
able primarily because it is a necessary constituent of the 
nucleo-proteins of every living cell. It is accumulated in 
relatively large amount in the seed, so in the younger 
stages of growth, when practically all cells are embryonic, 
it is relatively most abundant. 

Many observers have commented upon the prompt 
migration of the sum total of phosphorus compounds 
from maturing stems and other older vegetative parts 
to the growing tips or to the developing seeds. It has 
been shown by Wilfarth and his associates that during 
the ten days from June 17 to 27, as barley is maturing, 
there is a striking change in the phosphorus relations, 
the amount in the straw being reduced from 29.04 kilo- 
grams to 9.59 ; while in the grain there is an increase in 
the same time from 3.54 to 29.84 K. per hectare. At the 
same time they give data which they interpret to mean 
the movement of some phosphorus back into the soil. 

Loew was the first to suggest important additional 
functions of phosphorus. As a result of phosphorus 
hunger the cells of Spirog3 T ra soon cease to grow, but 
starch is formed for a time. He also found that oily and 
protein substances were not used, but in fact accumulated 
in the cell. Owing to the phosphorus content of lecithin, 
he explained the accumulation of fats by assuming that 
such substances are changed into lecithin before becoming 
assimilable by the protoplasm ; thus phosphorus would be 
essential in the assimilation of fats. 

Overton assumes that lecithin and similar bodies are 
important in the osmotic properties of the plasma mem- 
brane, this view being largely based upon the penetrability 



172 Plant Physiology 

of the membrane to substances like alcohol. There are, 
however, serious objections to this idea. Reed found, 
among other pathological conditions attending an insuffi- 
ciency of phosphorus, that starch was transformed into 
unusual carbohydrate forms, and that cell-walls were 
often thickened. 

93. The role of potassium. — Potassium is an essential 
element, and the experiments which have been carefully 
and accurately carried out make it possible to say that in 
general there may be no fairly complete substitution of 
potassium by means of the related metals, lithium, sodium, 
rubidium, and caesium, and generally very slight partial 
substitution among higher plants. It is, however, true 
that, when potassium in sufficient quantity is not available, 
the addition of sodium is almost invariably attended by 
increased growth. The relation to sodium is discussed 
more at length later. 

Potassium in organic food formation. — Many investiga- 
tors agree in assigning to potassium a peculiarly impor- 
tant function in the formation of carbohydrates and pro- 
teins. Loew and Reed have devoted special attention to 
this point. When potassium fails, starch is not formed, 
and even if sugar is furnished, proteins are not normally 
produced, ('ells in a condition to divide arc also consider- 
ably influenced by lack of potassium. Such cells might 
elongate to twice their normal length, supposedly by a 
process of stretching, but there would be no evidences of 
cell or nuclear division. Loew regards the potassium as 
a strong condensing agent (and he shows that in certain 
cases potassium is able to effect changes which sodium 
will not). Since condensation processes are probably 



Special Functions and Relations 



173 



involved in carbohydrate, fat, and protein making, the 
relation of potassium to general metabolism is deduced. 

The 'potassium and protein relation. — The relation 
between protein and potash in storage organs has also been 
shown to be suggestive, at least to this extent : Seeds or 
other organs rich in protein are generally relatively rich 
in potash, although there is no definite ratio. Loew cites 
certain analyses of Wolff which may be summarized in the 
following table : — 



Product 


No. 

Spe- 
cies 


No. 
Anal- 
yses 


Potash, Per 
Cent in Ash 


Protein, 
Per Cfnt 


Potash 
Avg. 
Per 
Cent 


Pro- 
tein, 
Avg. 
Per 
Cent 


Seeds of cereals 
Seeds of legumes 


5 

6 


200 
64 


16.32 to 31.47 
29.84 to 44.01 


9.8 to 11.0 
22.7 to 35.3 


23.07 

39.21 


10.2 
29.0 





The osmotic relation and winter injury. — It has been 
generally held that another important role of potassium 
may be found in its action as an osmotic agent. Some 
plants contain relatively large quantities of potash in their 
juices, as K 2 S0 4 , KN0 3 , KH 2 P0 4 , and certain organic 
salts. Other plants, however, have high osmotic coeffi- 
cients on account of organic substances, and there seems 
to be no sufficient reason why as an osmotic agent the 
potassium is to be regarded as having a constant role. 
In fact, the view seems to be justified that in so far as po- 
tassium is necessary osmotically it may be replaced by 
sodium. Certainly the high osmotic value of certain 
fungi which may grow upon strong sugar or nutrient solu- ' 



174 Plant Physiology 

tions is not due to the presence of K compounds, and this 
fact has been abundantly demonstrated. 

With this osmotic relation in view, it was natural that 
there should exist also the belief that plants afforded an 
abundance of potash are better able to withstand drought. 
This is not yet sufficiently proven. Resistance to drought 
may possibly be due in part merely to increased salt con- 
tent of the plant ; in which case, however, it would be 
inferred that many soluble salts should have a similar 
effect. The latter is not reported to be the case. The 
experiments of Atkinson in Alabama on the prevention 
of " rust " of cotton have been interpreted to mean that 
potassic fertilizers are partially important in the water 
relation of the plant, guaranteeing sufficient water, con- 
sequently preventing the blight, which is a combination 
of drought and fungous effects. Nevertheless, there is 
apparently no evidence that desert plants possess any 
particular relation to potassium. It is also claimed that 
by virtue of relations to the water-content plants well 
supplied with potash would be less injured by freezing. 

Maturity, quality, and color. — The belief is current 
that orchard trees well fertilized with potash ripen their 
wood more thoroughly, and that as a partial but direct 
consequence of this the shoots and buds are not so subject 
to winter or early spring injury. In other words, the belief 
indicates that potash content is a special factor in the har- 
diness of perennials. Heightened color and quality in 
apples has also been attributed to it, but a careful exami- 
nation of this point indicates that there is no such relation. 
It seems rather that such a deficiency of any element as 
to check growth necessarily affects quality. 



Special Functions and Relations 175 

94. The role of magnesium. — Magnesium is an ele- 
ment concerning some of the functions of which practically 
all physiologists seem to be agreed. It may be inferred 
that it does not play a direct role in the formation of pro- 
teins. It is, in general, more toxic to protoplasm than the 
other mineral nutrients, and according to Loew its chief 
function is probably to be found in the conveyance of 
phosphoric acid for assimilation. Magnesium is more 
abundant in those parts of the plant undergoing develop- 
ment, as in growing tips and seeds. This would imply 
that it acts indirectly to condition the formation of the 
nucleo-proteins. Loew believes that "the same amount 
of base can serve over and over again as the vehicle for 
assimilation of phosphoric acid." It is well known that 
magnesium is migratory in the plant, so that maturing 
organs are considerably depleted. Attention has been 
called to the fact that oily seeds contain a larger proportion 
of this element than do starchy seeds, and this is regarded 
as a point strengthening the argument of Loew respecting 
the function of this element, especially since lecithin is 
formed in cells rich in oil. Reed has also found that there 
is some definite connection between magnesium and phos- 
phorus. He has demonstrated that oil globules are not 
formed in Vaucheria when magnesium is lacking in the 
nutrient solution, and he believes that there is an intimate 
relationship between magnesium and vegetable oils. 

95. The role of calcium. — The judicious use of lime 
in plant production may be the determining factor in the 
active fertility of a soil. It appears that the addition 
of lime to soils is a practice which has shown more or 
less alternation in various agricultural epochs. Wheeler 



176 Plant Physiology 

has suggested a cause of this use and disuse. When the 
benefits from it at any time became known, this probably 
led -to excessive use, causing injury, whereby the practice 
again fell into disfavor. In the United States a careful 
study of the liming practice and of its effects has been 
made in comparatively recent years. 

Calcium has functions to perform which are strictly 
physiological; that is, directly important in the metabolism 
of the plant ; it has other effects distinctly ecological, 
affecting the plant through its action upon the physical 
and chemical environment. It is not always possible to 
distinguish the one form of effect from the other. From 
an agricultural standpoint Wheeler has given in the " Cyclo- 
pedia of American Agriculture " a concise enumeration of 
the effects. In this connection the physiological side re- 
quires more particular consideration. 

In vegetative organs. — There is generally a considerable 
accumulation of lime in leaves and other vegetative organs, 
and on this account it has been assumed to play an impor- 
tant role in some of the functions associated with the 
chlorophyll. Up to a certain point calcium hunger does 
not affect starch formation, and the evidence points rather 
to an inhibition of starch and other carbohydrate diges- 
tion and transport. In fact, many fundamental experi- 
ments have established a definite relation — w r hether 
direct or indirect it is impossible to say — between calcium 
content and starch digestion. The addition of soluble 
carbohydrates is generally beneficial where plants lack 
calcium. In this connection it is of interest to note that 
calcium is apparently not required by fungi and some 
of the lower algae, yet it is required by higher plants 
forming no starch. 



Special Functions and Relations 111 

Other investigators regard calcium as important, in 
the main, in the neutralization of oxalic acid and acid 
oxalates, assumed to be a factor in protein synthesis. 
Neutralization is often effected in this way, for calcium 
oxalate is of frequent occurrence ; yet in some of the 
higher plants there is no such accumulation of oxalates. 

Boehm considered calcium essential in the formation of 
the cell-wall, and while he erroneously interpreted this 
to be similar to the use of calcium in bone formation, yet 
the relation of an adequate calcium supply to the forma- 
tion of cell-walls has been clearly brought out by many 
investigators. This apparent function may be merely an 
indication of imperfect use of carbohydrates as above 
discussed. Moreover, the formation of complete cell- 
walls in the various fungi without calcium is against any 
supposition of its direct importance in modified cellulose 
formation. 

As early as 1880 it was ascertained that salts of magne- 
sium are toxic when used alone, and that this toxicity dis- 
appears when sufficient calcium is present in the nutrient 
solution. In recent times the peculiar and antagonistic 
relation which exists between calcium and magnesium, and 
also between other nutrient elements to a less extent, has 
been more completely developed. The work begun by 
Von Raumer, and followed up by Loew, Loeb, Kearney, 
and Osterhout, will be discussed at greater length under 
Balanced Solutions. It is necessary, at this time, merely 
to indicate that calcium is important in preventing the 
injurious effects of an excess of magnesium. 

Calcium in protein formation. — In studying this rela- 
tion of the elements, Loew has developed an important 



178 P^nt Physiology 

hypothesis respecting the role of calcium in protein forma- 
tion According to him we must anticipate a calcium- 
protein compound as important in the building up of the 
nucleus and plastids of the cell. In the absence of suffi- 
cient calcium he believes that magnesium takes its place, 
and that this magnesium compound does not possess the 
necessary capacity for imbibition phenomena required by 
the cell structures. There are some important objections 
to be met in considering this hypothesis, in view of the 




Fig 49. Effect of liming in the production of alfalfa ; no fertiliser :(i), 
lime only {2) , and lime with nitrogen (J) . [Photograph from the Rhode 
Island Exp. Sta.] 

facts that magnesium salts are not toxic for the fungi and 
for the lower alga, and, in the presence of small amounts 
of calcium, relatively nontoxic also for the marine algse, 
as well as for a few of the higher plants. 

On the other hand it is true that plants grown in solu- 
tions lacking calcium show, coincident with the expected 
pathological conditions, an increase in the magnesium 
content, whereas other pathological effects produced by 
unfavorable conditions show a normal ratio of calcium 
and magnesium. 



Special Functions and Relations 179 

Chemical effects. — Lime is almost as important through 
its action in rendering the soil environment chemically 
favorable as in its specific roles in cell metabolism. Soils 
in which vegetation is growing have, in general, a tendency 
to develop the condition fittingly termed acidity. When 
the acidity increases beyond a certain point, it may become 
extremely inhibitory to the proper growth of a variety of 
agricultural plants, and lime, either as carbonate or as 
slaked lime, is necessary in order to neutralize this condi- 
tion. The carbonate of lime is less injurious and more 
generally applicable in large quantities. 

The ecological relation of plants to soils containing 
much or little lime is particularly interesting, and has been 
extensively studied from the standpoint of the adaptability 
of crops and of the distribution of wild plants as well. 
Upon the crop side Wheeler has contributed excellent data. 
In general, the experiments indicate that when the soils 
show a marked acid tendency, liming is beneficial. 

Some of the plants to which the greatest benefit accrues 
are such as lettuce, beet, onion, and cantaloup. Again, 
crops such as cranberry, watermelon, red-top, cow-pea, 
and others may be favorably influenced when the acidity 
is considerable. The great majority of crops occupy an 
intermediate position, many responding satisfactorily 
under field conditions to moderate liming. Upon some 
Rhode Island soils the yield of sugar-beets has been in- 
creased by liming up to one hundred fold. Liming will 
also affect, within the season, the character of the weeds 
or native vegetation. It is of interest to note that closely 
related plants are differently affected; thus the watermelon 
and the muskmelon, or red-top and timothy, may be 



180 Plant Physiology 

contrasted, the last-named in each case enduring much 
less acidity. 

Lime is important in effecting a liberation of (by 
rendering available) other nutrients, and on this account 
it should be used cautiously, in order that waste by leach- 
ing may not result. 

It is important in maintaining phosphates in available 
form, and in counteracting the injurious effects of many 
substances in the soils, including certain products of fer- 
tilizers. In many ways it has an intimate relation to the 
nitrogen supply of plants, for it promotes the formation 
of nitrates from organic matter, diminishes the destruction 
of these, and seems to be generally almost indispensable 
for the proper development of the nitrogen-fixing root- 
tubercle organisms. 

The above effects may be considered those of most 
intimate consequence for plants generally ; but in addition 
it may improve (by rlocculation) heavy soils, and it may 
be important as an insecticide and a fungicide (although 
it is favorable to potato scab and to root rot of tobacco). 

96. Iron. — A certain amount of iron seems to be 
necessary as one of the factors in the normal development 
of leaf green, or chlorophyll, although it is not regarded 
as a constituent of the organic bodies which make up this 
substance. Lack of iron is one of the many conditions 
leading to pathological chlorosis. It may be that the lack 
of iron affects the protoplasmic structure (the plastid) 
in which the chlorophyll is deposited, for the best evidence 
points to the use of iron by every living cell, including, 
therefore, those organisms which contain neither this pig- 
ment nor any allied compounds. 



Special Functions and Relations 181 

In cases where iron is deficient in the soil, or held as 
markedly insoluble compounds, beneficial results have 
been obtained by the application of a soluble salt. Rich- 
ards and Ono have shown that iron salts have a remarkably 
stimulating effect upon filamentous fungi, increasing the 
dry weight several fold over that obtained when the mini- 
mum used is merely that which would occur as impurities 
in the purest salts. Final proof of the relation of diverse 
plants to iron is most difficult to obtain, owing to the pres- 
ence of traces of this metal in many of the purest salts. 

97. Sodium. — Sodium, a metal indispensable in ani- 
mal nutrition, is not required by plants. It would seem 
that it may at times prove beneficial, and in the field 
relations of crops it is often indirectly serviceable by setting 
free other requisite bases. Breazeale has shown, by experi- 
ments interesting both with respect to method and result, 
that more sodium is absorbed, and that it may be directly 
beneficial, in the absence of sufficient potassium. 

Wheeler has conducted extensive field experiments, 
upon various aspects of the sodium problem. This work 
supports the views advanced, in a measure. It also indi- 
cates that field applications of sodium may be beneficial 
in subsequent years " in those cases where the previous 
application of potassium salts had been large." He re- 
gards this as " due, in part at least, to the retention in the 
soil of a part of the previous applications of potassium 
salts, by virtue of extra soda having been taken up by the 
preceding crops in the place of superfluous potash, whereby 
the potash supply in the soil was really conserved." 

98. Chlorine. — Chlorine seems to be generally ines- 
sential for the complete development of the higher plants. 



182 Plant Physiology 

Knop and other students of nutrition so regarded it, and 
it is sometimes omitted from the nutrient solution. It is 
an invariable constituent of the soil solution, and either 
on this account, or in the belief that it is generally some- 
what advantageous, it is commonly added to the nutrient 
ration as NaCl or KCL 

Nobbe and others have found KC1 indispensable in the 
proper maturity of buckwheat, which, deprived of it, 
develops a pathological condition at or following the period 
of flowering, resulting in a failure to form seed. A light 
fertilization of special crops with sodium chlorid has not 
infrequently resulted in increased yield ; but in most cases 
it is not certain that the action is direct, and even less clear 
that it is the additional chlorine which is important in the 
substance employed. This relation of plants to chlorine 
is the second notable difference in the metabolism of 
plants and animals. 

99. Sulfur. — Sulfur is primarily important because of 
the fact that it is contained in albuminoidal compounds. 
It occurs in some of the by-products of protein production, 
and also as sulfates of the bases — especially potassium 
— occurring in the cell-sap. It is usually required in such 
limited quantity that the seed may furnish all that is 
needed for the normal growth of the plant through a con- 
siderable period. 

100. Silicon. — Silicon forms a predominant part of the 
ash of many grasses and other plants. It accumulates in 
old stems and caulms, and may constitute from 40 to 
70 per cent of the ash of cereal straws and corn stover. 
Nevertheless, corn may be grown without any further 
addition than that furnished bv the seed. One of the 



Special Functions and Relations 183 

species of the scouring rush (Equisetum) has an ash con- 
tent of Si0 2 , amounting to from 70 to 80 per cent. The ac- 
cumulation is chiefly in the cell-wall, where it is doubtless 
important in support and protection. Silicon is regarded 
as an inessential element because development proceeds 
in its absence ; but in the complex relations of plants in 
the field it may determine the capacity of a plant to exist 
in a particular habitat. Wolff regarded silicon as impor- 
tant in furthering the migration of phosphoric acid com- 
pounds from maturing leaves and stems to the forming 
seeds. 

References 

Breazeale, J. F. The Relation of Sodium to Potassium in 

Solution Cultures. Journ. Amer. Chem. Soc. 28 : 1013- 

1025, 1906. 
Deleano, N. T. Etude sur le role et la fonction des sels mine- 

raux. 48 pp., 1907. 
Loew, O. Ueber die physiologischen Funetionen der Calcium 

und Magnesiumsalze im Pflanzenorganismus. Flora. 75 : 

368-394, 1892. 
The Physiological Role of Mineral Nutrients. Bur. Plant 

Ind., U. S. Dept. Agl. Bui. 45 : 70 pp., 1903. 
Reed, H. S. The Value of Certain Nutritive Elements to the 

Plant Cell. Ann. Bot. 21 : 501-543, 1907. 
Raumer, v. Calcium und Magnesium in der Pflanze. Landw. 

Versuchsst. 19 : 253-280, 1883. 
Woods, A. F The Relation of Nutrition to the Health of 

Plants. Yearbook U. S. Dept. Agl. (1901) : 155-176, 7 pis. 

Texts. Jost, Pfeffer. 



184 Plant Physiology 

BALANCED SOLUTIONS 

Since the early studies upon the mineral nutrients of 
plants, it has been more or less apparent that any one of 
the nutrient salts employed singly may be injurious, or 
may inhibit growth. The extent of this inhibition of 
growth has in recent years been more extensively meas- 
ured. Moreover, it has long been realized that in the 
preparation of the nutrient solution a certain ratio of the 
different salts is required, or may be favorable, for the best 
results. 

It is now known that there are certain interesting antag- 
onistic relations between some of the nutrient and other 
bases whereby the inhibitory effects of one may be in part 
or entirely counterbalanced by the presence of another. 
A solution in which the inhibitory or toxic action of one 
substance is rather effectually eliminated by an " antago- 
nistic " compound is now generally termed a balanced solu- 
tion. Some cases of alleged antagonism are apparently 
complicated by factors of nutrition and exosmosis, but 
at present it is not possible to evaluate the different 
factors. 

101. The injurious action of certain basic nutrients. — 
Although toxic action in general is discussed at length 
later, it is necessary here, in connection with balanced so- 
lutions, to note the relations of some plants to some of the 
several single nutrient compounds. The following table, 
from data by Kearney and Harter, shows approximately 
the limiting concentrations of two sodium and two mag- 
nesium salts, endured for twenty-four hours by wheat, 
lupine, and maize : — 



Special Functions and Relations 



185 





Wheat 


Lupine Maize 


Salt 


Parts of 
a Normal 
Solution 


Parts 

per 

100,000 

of 
Solution 


Parts of a 
Normal 
Solution 


Parts per 
100,000 

of 
Solution 


Parts of 

a 
Normal 
Solution 


Parts per 
100,000 

of 
Solution 


Magnesium sulfate 
Magnesium chlorid 
Sodium sulfate . . 
Sodium chlorid . . 


.007 
.009 
.043 
.054 


39 
108 
302 
313 


.00125 
.0025 
.0075 
.02 


7 

12 

53 

116 


.25 

.08 
.05 
.04 


1400 
394 
353 
232 



In general, the magnesium compounds are particularly 
toxic to the higher plants. Corn is an apparent exception 
to the rule, as are also most fungi and some algae. On 
account of the fact, then, that magnesium compounds are 
so generally harmful when alone, or in relative excess, it 
is of special interest to note in some detail the relation of 
this element to other bases. 

102. The relation of calcium to magnesium. — The 
toxic action of magnesium and the effect of calcium in 
modifying it were established by Von Raumer in 1883 ; 
but the most important work in outlining and directing 
attention to this field with respect to plants was done by 
Loew and his associates. There is at present a mass of 
data available both on the plant and on the animal side. 
As a general result of all the work on the higher plants it is 
now clear that when magnesium is injurious, the presence 
of calcium in a certain ratio (variable for the plant) de- 
stroys this toxicity entirely. To explain this relation Loew 
formulated his theory of the existence of a calcium-protein 
body as previously outlined (section 95). 



186 Plant Physiology 

The lime-magnesia relation in the soil is, moreover, of 
much practical importance, and it is certain that when 
magnesia is relatively abundant in soils there is usually 
need of liming. Under such circumstances it is obvious 
that the application of a dolomitic limestone (rich in mag- 
nesium) should be avoided. The evidence upon this 
point is also extensive. There are practical difficulties, 
however, in determining the proper ratio of CaO : MgO in 
the soil, for the question of availability must be considered. 
Water culture and pot experiments have suggested such 
differences in the requirement of plants as shown by the 
following favorable ratios : — 

Buckwheat CaO to MgO 3 to 1 

Cabbage CaO to MgO 2 to 1 

Oats CaO to MgO 1 to 1 

Loew believes that the greater the leaf surface produced in 
a given time, the greater the necessity for lime; that is, 
the higher the ratio. In this connection it is shown that 
with respect to the lime requirement the need of tobacco, 
clover, grass, sugar-beet, and wheat form a decreasing 
series. The cereals never show a ratio higher than 2:1. 
In solution cultures in the laboratory the effect of the omis- 
sion of calcium from a solution containing magnesium, 
or the inappropriate ratio of calcium, results in a markedly 
decreased growth. In the following chart, giving the 
effect of the calcium-magnesium ratio upon the growth 
of the Canada field pea, this fact is made clear, 1 the 

1 These data correspond to the series of cultures in Figure 51, and are 
from careful laboratory experiments by graduate students. 



Special Functions and Relations 



187 



single average plant being taken as the basis of the dia- 
gram, — each major ordinate denoting .1 gm.:- — 




Fig. 50. Antitoxic action of calcium and magnesium nitrate ; curves of 
total green weight (unbroken line) and green weight of stem (broken 
line). 

In the cultures diagramed above the whole period of 
growth is 17 days. The original quantity of the solution 
was maintained by replacing with distilled water the loss 
by transpiration; but after 14 days all solutions were 



188 Plant Physiology 

completely renewed. For such a period of growth the 
seeds of the pea carry a fairly adequate supply of the other 
nutrients. Again, the seed is relatively rich in magnesium, 
so that the omission of this element affects growth very 
slightly, while the same fact emphasizes the need of cal- 
cium in the solution. 

103. Other nutrient bases and antitoxic action. — The 
neutralizing action of various bases upon one another 
has been demonstrated by Loeb, Kearney, Osterhout, 
and others. In this regard calcium is most important. 
At suitable concentrations it reduces the toxicity of delete- 
rious solutions containing either potassium, sodium, or 
ammonium, as well as of certain nonnutrient bases. 

The table on the opposite page includes data furnished 
by McCool 1 from two distinct series of cultures with the 
Canada field pea grown 30 days. 

In the first series the concentration of sodium employed 
shows no growth whatever, and the addition of one 
fortieth as much calcium gives a very considerable growth ; 
therefore, a marked antitoxic effect. The best growth 
occurs where the stronger concentrations of calcium are 
used with the sodium, so that there appears to be a slight 
mutual antagonistic action with respect to peas. The 
strong effect of calcium upon the relatively toxic ammo- 
nium salt is apparent. 

In general, all of the nutrient bases show a series of rela- 
tions with respect to toxic action upon plants. For each 
base the relations may be different, and a certain varia- 
bility is to be accounted for by differences in the composi- 

1 The experiments of which these constitute a small part will be pub- 
lished as a bulletin of the Cornell Experiment Station. 



Special Functions and Relations 189 

Antitoxic Action — Canada Field Peas 



Solution Cultures 
Contain 



Average Length of 
10 Plants 



Tops, 
in cm. 



Roots, 
in cm. 



Green Wt. in Grams 
of the 10 Plants 



Tops 



-^-CaCl 2 

1000 

-N-CaCl 2 

2000 

-N-CaCl 2 

4000 

-^NaCl " . . 

100 

-^- CaClo and — NaCl 
1000 100 

-^- CaCU and -^- NaCl 
2000 " 100 

-^- CaCl 2 and — NaCl 
4000 100 

Distilled water 

-^ CaCl 2 

100 

-^-NH 4 C1 

2000 

-^- NH 4 C1 

3000 

-^-NH 4 C1 

4000 

-^- CaCl* and -^- NH 4 C1 
100 " 1000 

-^- CaClo and -^- NH 4 C1 
100 " 2000 

-^- CaCL and -^- NH 4 C1 
100 3000 

-^- CaCl 2 and -^- NH 4 C1 
100 4000 

Distilled water 



18 
19 

no growth 
18 
25 
17 



13 
10 

9.5 
no growth 
12 ' 
14 
11 

3 



6.2 
5.1 
4,8 
no growth 
5.25 
6.5 
4.8 
2.2 



3.3 
2.2 
2.1 
no growth 
3.9 
4.6 
3.7 
1.8 



13 

4 
4 
4 
9 
11 
10 
10 



8 

no growth 
no growth 
no growth 

7 
8 
8 
8 
3 



2.85 
no growth 
no growth 
no growth 

2.0 

2.0 

2.54 

2.03 

1.8 



190 



Plant Physiology 




Special Functions and Relations 191 

tion or absorptive action of the plants used as indicators. 
Moreover there is diversity in the visible results of the 
toxic action by the different nutrients ; thus salts of 
ammonium kill the roots before the shoots are noticeably 
affected, while sodium salts kill the shoot promptly. It 
is important that there is toxic action, and that there may 
be antagonistic or mutually antagonistic action. 

An entirely satisfactory explanation of these phenomena 
is not at present available. Loew's views on the calcium- 
magnesium relation are strengthened by the opinions of 
Loeb and others who postulate in organisms a number of 
metal proteins, so that any solution containing only one 
class must be ultimately toxic. Even this view is not suffi- 
ciently broad to account for all of the known facts ; for 
example, the antagonism between inessential and essential 
ions. 

LABORATORY WORK. — SUGGESTED EXPERIMENTS 

Injurious action of single nutrients. — After consulting the 
literature of the subject determine the concentrations of MgCl 2 , 
CaCL, KC1, and NH 4 C1, which when used separately will just 
permit the growth of roots of wheat or peas. Employ ten 
plants in tumblers or bottles, as in the nutrition studies. 

Balanced Solutions. — Guided by the indications given under 
nutrient solutions regarding manipulation, set up the experi- 
ments outlined below, employing in each culture wheat or 

N 
Canada field peas. Prepare stock cultures of — Ca(N0 3 ) 2 , 

50 

— Mg(NO»)o, — NaCl, — CaCL, also double strength of nutri- 
50 100 100 

ent solution ; from these prepare, by dilution, all the following 
cultures, also employing distilled water as a check : — ■ 



192 



Plant Physiology 



Calcium versus Magnesium 



9. 

I 125 cc. 
10. Distilled 



Use 

. 4 Ca(N0 3 ) 2 . 
50 

. distilled water. 

. ^ Mg(N0 3 ) 2 . 

50 
. distilled water. 

.^Mg(N0 8 ) 2 . 

. distilled water. 

.|?Mg(N0 3 ) 2 . 

. distilled water. 

. ^ Ca(N0 3 ) 2 . 
50 

. ^ Mg(N0 3 ) 2 . 
50 

.^Ca(N0 3 ) 2 . 

,^Mg(N0 3 ) 2 . 

50 
i. distilled water. 

►. -^ Ca(N0 3 ) 2 . 
50 

5 .^Mg(N0 8 ) 2 . 

5. distilled water. 

j. ^ Ca(N0 8 ) 2 . 
50 

i. nutrient sol. 

. N- 

nutrient sol. 
H 2 0. 



^Mg(N0 3 ) 2 . 



100 

100 

iL 

250 



Solution Contains 



Ca(N0 3 ) 2 . 



Mg(N0 3 ) 2 . 
Mg(NO a ) 2 . 



^ Mg(N0 3 ) 2 . 
500 5 



N 



ii- Ca(N0 3 ) 2 and £- Mg(N0 3 ) 2 . 

100 100 



100 



Ca(NOs) 2 and ^ Mg(N0 3 ) 2 . 



100 



Ca(N0 3 ) 2 and ^ Mg(N0 3 ) 2 



— Ca(NOs) 2 and nutrient sol. 
100 



— Me(NOs) 2 and nutrient sol. 
100 

Distilled water. 



Special Functions and Relations 193 

The above series should show toxicity of the magnesium salt 
and antagonism. 

In the same manner as for the preceding prepare solutions 
to contain the following : — 



N x Tn n, i„ N 



n [||CaCl 2 . 

alld < XT 12 ' ^ CaCl2 ' 13 ' ^ NaCL 14 - dL NaC1 - 

Ha. I N_ C aCl2. 250 ° 10 ° 2500 

15. ^CaCl 2 and -^- NaCl. 18. -^- CaCl 2 and -g- NaCl. 
25 100 2500 2500 

16. S CaCl 2 and -§- NaCl. 19. ^- CaCl 2 and nutrient sol. 
^o ^oOO ^o 

17 '• ?JL CaCl 2 and -^- NaCl. 20. -^- NaCl and nutrient sol. 
2500 100 100 

Permit the plants to grow under favorable conditions four- 
teen days, supplying distilled water as needed by transpiration. 
Discuss the results, tabulate data on length and green weight ; 
also plot curves of the total green weight in the two series. 



References 

Duggar, B. M. The Relation of Certain Marine Algae to Vari- 
ous Salt Solutions. Trans. Acad. Sci., St. Louis. 16:473- 
489, 1906. 

Hansteen, B. Ueber das Verhalten der Kulturpflanzen zu den 
Bodensalzen. Jahrb. f. wiss. Bot. 47 : pp. 289-377, 1 pi., 
19 figs., 1910. 

Harter, L. L. The Variability of Wheat Varieties in Resist- 
ance to Toxic Salts. Bur. Plant Ind., U. S. Dept. Agl. Bui. 
79:48 pp., 1905. 

Kearney, T. H., and Cameron, F. K. Some Mutual Relations 
between Alkali Soils and Vegetation. U. S. Dept. Agl. 
Report. 71 : 78 pp., 1902. 

Kearney, T. H., and Harter, L. L. The Comparative Toler- 
ance of Various Plants for the Salts Common in Alkali Soil. 



194 Plant Physiology 

Bur. of Plant Ind., U. S. Dept. of Agl. Bui. 113 : 22 pp., 
1907. 

Loeb, J. Toxic and Antitoxic Effect of Ions. Studies in Gen- 
eral Physiology. Part II, Chap. 35, etc., 1905. 

Loew, O., and May, D. W. The Relation of Lime and Magnesia 
to Plant Growth. Bur. Plant Ind., U. S. Dept. Agl. Bui. 1 : 
53 pp., 1901. 

Loew, O. The Physiological Role of the Mineral Nutrients. 
Bur. Plant Ind., U. S. Dept. Agl. Bui. 45 : 70 pp., 1903. 

Osterhout, W. J. V. The Importance of Physiologically Bal- 
anced Solutions for Plants. Bot. Gaz. 42 : 127-134, 1906 ; 
Ibid. 44 : 259-272, 7 figs., 1907. 

Die Sehutzwirkung des Natriums fur Pflanzen. Jahrb. f. 

wiss. Bot. 46 : 121-136, 3 figs., 1908. 



CHAPTER IX 

THE INTAKE OF CARBON AND THE MAKING 
OF ORGANIC FOOD 

Organic matter constitutes the predominant part of the 
solid constituents of plants. As organic matter so-called, 
this element is linked chiefly with hydrogen ; with hydro- 
gen and oxygen ; with hydrogen, oxygen, and nitrogen ; 
or with the preceding and one or more of the essential 
mineral elements. Nevertheless, carbon may be regarded 
as the significant element of organic compounds. The 
number of such organic compounds in plants and animals 
and their products is almost beyond count. It is the 
special province of organic chemistry to deal chemically 
with these carbon series, but some account of the origin, 
nature, and role of certain of these substances in the living 
organism is a most important part of physiology, whether 
elementary or advanced. 

104. The amount of carbon in the plant. — We have 
noted the relatively small amount of mineral or ash ele- 
ments in plants, constituting usually from 1 to 5 per cent 
of the total weight of the water-free substance. Nitrogen 
adds a further fraction, — seldom more than 5 percent, 
— and the remainder, that is, more than 90 per cent of 
the dry matter, is made up of carbon, hydrogen, and oxy- 
195 



196 Plant Physiology 

gen. A crude picture of the distribution and importance 
of the carbon in plants is afforded by the well-known 
process of charcoal making, — a burning without free 
access of oxygen. As a result, the hydrogen, nitrogen, 
and oxygen of the plant are set free, while only carbon 
and the small proportion of ash remain. When burned 
with abundant ox} r gen, the carbon combines with the 
oxygen by an oxidation or hydroxylation process, and 
gaseous carbon dioxid is ultimately formed. In this case 
a perfectly definite amount of energy, as heat, is released. 

105. Carbon dioxid the source of carbon in green 
plants. — We are now concerned with the source from 
which plants obtain their carbon supply, the conditions of 
intake, and the method by which the carbon is incorpo- 
rated into food for the plant cell. Carbon in inorganic 
form, especially as carbonates of lime and magnesia, con- 
stitutes no inconsiderable portion (about twice as much as 
phosphorus) of the minerals of the earth's crust. Yet, as 
will be subsequently indicated at length, carbonates are 
valueless as a source of the carbon from which to make 
organic compounds for either plant or animal. Moreover, 
both water and sand cultures abundantly demonstrate 
that green plants are able to grow and to attain their full 
development (necessitating the making of organic matter) 
in nutrient solutions containing no carbonates and also 
no organic matter whatsoever save that derived from the 
seed. 

It is easy to convince ourselves of the requirements of 
common animals with respect to organic matter. They 
feed upon plants or other animals, or upon the products 
of these. Moreover, there are the countless fungi and 



The Intake of Carbon 197 

bacteria (with marvelously few exceptions l ) familiar 
as molds, plant parasites, mushrooms, organisms of decay, 
and of various fermentative processes ; these all require 
an intake — from without the body — of organic carbon 
nutrients. Organisms which thus obtain their carbon as 
organic matter, and which have no apparatus for making 
it from carbon dioxid and water are in the end dependent. 
Such organisms have also been termed heterotrophic. 
Green plants are practically alone in being able to make 
organic matter out of the raw materials, carbon dioxid and 
water; they are independent. Conveying the idea that 
they make organic food somewhere within the body, and 
in the first instance for their own use, they have been called 
autotrophic. 

106. Chlorophyllous plants. — So far as is known, green 
plants have always supplied the earth with organic matter, 
including fuel. The leaf-green which they contain is the 
strongest link binding living things to the sun, — the one 
ultimate source of radiant energy available upon the earth. 
The means whereby this making of organic food is accom- 
plished is fundamentally important, and requires careful 
consideration. All living processes and phenomena are 
important, but since this stands out as the method whereby 
the world's supply of organic matter is made, the process 
assumes an interest scarcely second to that of life itself. 

The green or yellow-green color, sometimes partially 
veiled, is practically universal among plants which we now 
recognize as possessing the highest type of plant habit 

1 The exceptions consist in a few species of bacteria, subsequently 
discussed (section 128), whose paltry contribution to the stupendous 
quantity of organic matter existent is such as to be wholly negligible. 



198 



Plant Physiology 



and form. It characterizes also to a very high degree the 
algse and mosses, but it is absent from all the fungi. Sev- 
eral facts regarding the habi- 
tats and distribution of green 
plants afford us an indication 
of some of the conditions req- 
uisite for the proper work 
of plants thus endowed. The 
presence of the green color 
referred to is universally indic- 
ative of the possession of 
chlorophyll, a mixed pigment, 
imbedded in certain chlo- 
roplasts, or chlorophyll con- 
taining bodies which are differ- 
entiated portions of the living 
protoplasm. In particular, it 
is apparent that plants con- 
taining this substance are sun- 
loving or at least light-loving 
organisms. They may grow 
in partial shadow at times, but 
they are wholly absent from 
all permanently dark or deeply 
shaded places. The large sur- 
faces of the leaves and the evident arrangement of these 
and of the branches which bear them, with respect to light, 
all indicate clearly a certain relation of green color with the 
light factor. As the chief bearers of chlorophyll in seed- 
plants the leaves command special attention, wholly aside 
from their other functions or accessory work. Any agency 




Fig. 52. Cell of chlorenchyma 
showing chloroplasts with starch 
grains. [Adapted.] 



The Intake of Carbon 



199 




200 



Plant Physiology 



affecting their health injuriously, such as insect pests, 
fungous diseases, general unfavorable environment, ac- 




Fig. 54. Abutilon, side view, under greenhouse illumination. 



cumulation of dust or soot thereon, means restriction of 
their work in production. 

107. Respecting the distribution of chlorophyll. — In 
the higher plants the chlorophyll bodies may be disposed in 



The Intake of Carbon 



201 



all exposed vegetative organs, but the leaves are primarily 
the seats of their occurrence. The palisade and general 
parenchyma cells of the leaf ordinarily contain many 




Fig. 55. Abutilon ; looking down upon the plant shown in Fig. 54. 

chloroplasts (Fig. 25). Such tissue is designated chlor- 
enchyma. In stems or other thick organs the chloren- 
chyma is comparatively near the surface ; for, as a rule, the 
formation of chlorophyll is directly or indirectly dependent 



202 Plant Physiology 

upon light. Notice the color of seedlings which have 
grown in the dark and of grass beneath a board or pile of 
leaves. The epidermal cells of seed-plants are commonly 
colorless, yet the guard cells of the stomata are important 
exceptions. Nevertheless, there are certain structures 
which, supplied with a good food-supply, contain chloro- 
phyll, even when produced in the dark ; for example, the 
cotyledons of pine. 

The white or yellow areas of variegated leaves may 
contain no chlorophyll ; but leaves which are during 
growth brown, red, or otherwise highly colored contain 
chlorophyll bodies, the color in such cases being veiled by 
the presence of other pigments often present in the cell- 
sap. The diverse pigments of many algae exhibit a 
greater complexity. In the great majority of plants the 
chlorophyll bodies are discoidal or button-like forms (often 
lenticular or more nearly plano-convex), although in cer- 
tain of the algae (Spirogyra, desmids, etc.) they may 
possess unusual peculiarities of shape. The intimate 
structure of the chloroplast is none too well known. 
Briefly, it may be said that there is a cytoplasmic stroma, 
and within this is contained the green pigment, somewhat 
diversely deposited in different cases. 

108. The nature and properties of chlorophyll. — By 
means of alcohol, chlorophyll may be extracted from plants, 
leaving the tissues practically white. Either ethyl, 
methyl, or denatured alcohol may be employed, and the 
process is greatly facilitated by carefully bringing the 
alcohol to a temperature close to its boiling point over a 
water-bath. Seedling plants of the horsebean, small ce- 
reals and grasses, radish, or nasturtium afford as favorable 



The Intake of Carbon 203 

solutions as may be conveniently obtained. The solution 
is fluorescent by reflected light, and it is rapidly decom- 
posed in strong light. 

The chlorophyll pigment as extracted is a mixed sub- 
stance. Two products which are constant and predomi- 
nant permit of partial separation through their diverse 
relations to solvents. Thus if benzole is added to the 
alcoholic solution and the latter vigorously shaken, there 
will result on standing a blue-green benzole layer and a 
yellow alcohol layer. There are therefore two substances, 
a blue-green one which has passed largely into the benzole, 
and from the color it is usually called blue chlorophyll 
or cyanophyll ; while the yellow substance remaining in 
the alcohol is mostly carotin. 

Apparently cyanophyll does not exist alone in nature. 
It is a complex molecule containing nitrogen, and is vari- 
ously supposed to have phosphorus or magnesium associ- 
ated or combined with it. Cyanophyll is closely related, 
it would seem, to the haemoglobin of blood ; and it yields 
a variety of decomposition products, some of which col- 
orimetrically and chemically seem to be identical with 
certain products of haemoglobin. 

Carotin is of common occurrence in a variety of colored 
tissues, and in its crystalline form it is most conspicuous 
in the root of the carrot and in the petals of certain orange 
or yellow flowers. This pigment belongs to the group 
often called xanthophyll. The term "etiolin" is also ap- 
plied to it. The substance is present in etiolated organs, 
and it may long persist in the chloroplasts of leaves during 
the autumn. 

The most important property of chlorophyll is its ca- 



204 Plant Physiology 

pacity to absorb light, that is, radiant energy to which the 
retina of the eye is sensitive. The relation of the chloro- 
phyll and of its main constituents to the absorption of 
light of different wave lengths, as shown by a spectro- 
scopic examination, is discussed later. The radiant energy 
absorbed by the chlorophyll is the force operative in photo- 
synthesis. 

109. The factors essential in photosynthesis. — We 
may now review briefly the essential features of the process 
whereby chlorophyll-containing plants in the presence of 
light are able to construct organic food-materials. The 
process is termed photosynthesis. In order that photo- 
synthesis may proceed in the cells of healthy plants, it is 
necessary that light shall fall upon chlorophyll bodies in 
the presence of aqueous carbon dioxid. Temperature and 
other factors are important, — the exact relation to tem- 
perature being especially difficult to anatyze, — and in 
general the process is possible only within a certain range 
of physiological conditions. Nevertheless, under ordinary 
conditions of growth we may regard as the primarily 
essential factors: (1) chlorophyll, (2) light, and (3) carbon 
dioxid, the last two of which will receive further consider- 
ation later. 

110. The course of photosynthesis. — Briefly stated, 
the gas exchange and the actual phases (several of which 
are more or less simultaneous) of the process of photo- 
synthesis as commonly conceived are as follows : — 

1. Gas exchange between the green tissues and the 
surrounding air, whereby carbon dioxid may be absorbed 
by the cell-sap and reach the protoplasm. 

2. The absorption of radiant energy, as light, by means 
of the chlorophyll bodies. 



The Intake of Carbon 205 

3. The use of this kinetic energy in the decomposition 
of carbon dioxid and water (H 2 + C0 2 , or H 2 C0 3 ), the 
synthesis of an elementary organic product, and the con- 
sequent storage of potential or latent energy. 

4. The probable condensation of the synthate into a 
carbohydrate of high food value, generally fruit sugar, 
which is then often in part transformed into starch. 

5. The elimination, by gas exchange, of 2 , a by-product 
of the process (some of which, however, may be used in 
respiration, subsequently treated). 

It is seen, therefore, that there is a physical mechanism 
for gas exchange, a series of transformations of energy and 
of compounds, and ultimately the deposition of food-ma- 
terials, frequently starch. It is now necessary to consider 
a method of demonstrating this process, and later there 
will be required a further consideration of the course of 
events, the factors involved, the energy transformations, 
and some of the products resulting. 

111. The demonstration of photosynthesis. — It is 
possible to demonstrate photosynthesis in any plant more 
or less completely by one or more of several methods, and 
no single simple experiment will reveal all the facts desired. 
With all other factors well controlled, 1 increase in weight, 
or the accumulation of some organic product (especially 
starch) are practicable demonstrations. Another type of 
experiment involves, when accurate, an analysis of the 
gas used, or that eliminated, or both ; but such eudio- 

1 The student who may pursue this matter farther should examine 
carefully the difficulties and beauties of well-controlled experiments, 
consulting Ganong's "Plant Physiology" (2d Ed.), pp. 79-114; also the 
earlier account in Sachs. 



206 Plant Physiology 

metric methods should be carried out in an accurate man- 
ner by the use of special apparatus. 

For demonstration purposes the evolution of oxygen 
from cut stems of water plants (such as Elodea or Ca- 
bomba) is the simplest indication of photosynthesis ; but 
this may not be applied to land plants. During photo- 
synthesis gas escapes from the large air chambers through 
the cut stems, and with vigorous action a slow stream of 
bubbles may arise. It is then necessary to employ a 
method whereby these bubbles may be caught so that the 
gas may be simply identified. 

A funnel may be inverted over a quantity of clean, 
growing sprigs of Elodea, or water weed, in a deep vessel 
or aquarium. Over the funnel is inverted a test-tube of 
water (Fig. 58) for the collection of the oxygen. In order 
that there may be free access of carbon dioxid, the funnel 
should be much smaller than the vessel and should rest on 
supports several inches above the bottom ; while the 
water should be spring or well-aerated tap water, or should 
contain a supply of C0 2 introduced from a generator. 
The gas caught in the tube may be tested by proper ma- 
nipulation (see Laboratory experiments, p. 221), with an 
oxygen absorbent, preferably pyrogallate of potassium. 

If the gas is collected under favorable conditions, it will 
consist largely of oxygen, — about four fifths ; the remainder 
consisting of other gases formed in the plant and of nitro- 
gen from the air, which must, of course, diffuse into the 
air spaces. Having determined that oxygen is the chief 
part of the gas given off as bubbles from water weeds, 
during photosynthesis, the simple bubble-counting method 
may be employed in determining relatively the rate of 



The Intake of Carbon 207 

photosynthesis with different intensities and quantities 
of light, with varying quantities of C0 2 (up to the satura- 
tion point of water), at various temperatures, etc. 

112. The formation of sugar and starch. — With respect 
to the formation of organic food-material it has been indi- 
cated in the brief outline of the course of photosynthesis 
that glucose is generally regarded as the first stable result. 
The formation of glucose and free oxygen from carbon 
dioxid and water constitutes a complex process, but the 
reaction is commonly expressed in the following conven- 
tional manner : — 

6 C0 2 + 6 H 2 = CeHjA + 6 2 . 

Some years ago the view was advanced by Von Baeyer 
that formaldehyde is an early step in the reduction of the 
carbonic acid, and that then six molecules of the formalde- 
hyde, H . COH, become linked together or condensed to 
form a hexose sugar, C 6 H 12 6 . Recent work along many 
lines strengthens this conception of the process, and it 
seems to have been demonstrated (although there are some 
criticisms of the method) that by artificial experiments 
with the factors light, chlorophyll, and C0 2 , formaldehyde 
may be produced, although in small amount as compared 
with the quantity which must result from photosynthesis. 
The details of this work, however, and the criticisms 
thereof do not require consideration. 

It is difficult to picture simply all possible relations of 
the glucose which may appear as the first stable product, 
but the accumulation of this substance in the cell leads to 
the formation of other sugars, especially bioses (C^H^On) 
and ultimately to starch, a complex molecule having the 
general formula (C 6 H 10 O 5 ) w . 



208 Plant Physiology 

Starch then is an accumulation product apparently 
conditioned only when the tension of the sugar which has 
been produced is considerable, 'at least so considerable 
that the cell is unable to use the surplus in building up the 
permanent structures, or to remove it fast enough. Starch 
is deposited within the chloroplasts in the form of small 
granules. During the growing season it normally accu- 
mulates in most leaves through the day, or so long as the 
leaves are exposed to strong light ; while during the photo- 
synthetic inactivity of the night much or all of this starch 
may be removed. In most cases the leaf will be depleted 
of starch if placed in the dark for a period of 12 hours, if 
the leaf is not in itself a storage organ. The process of 
starch removal and subsequent deposition, when that 
occurs, invites special consideration later. 

In those plants forming starch abundantly in the leaves 
it is often desirable, and extremely convenient, to employ 
the relative accumulation of starch as a rough qualitative 
indication of photosj-nthetic activity. Leaves from which 
chlorophyll has been extracted may be stained with a 
weak alcoholic solution (tincture) of iodine, the leaves 
being preferably placed on a white plate to be stained. 
When added to a weak suspension of starch, or to a weak 
starch paste, iodine yields an intense blue or blue-black 
color. Starch in the leaf, or in other tissues, is, however, 
considerably obscured, and it often gives a blue-brown 
or even brown-black reaction. Plants of the iris, lily, 
amaryllis, and orchid families form, as a rule, little or no 
starch. 

113. The diffusion process. — It has already been 
shown that the leaf (or an analogous structure) is an ad- 



The Intake of Carbon 209 

mirable device to permit rapid diffusion with a minimum 
direct exposure of delicate cells. Uncutinized surfaces are 
moist and may absorb C0 2 directly, but the epidermis is 
usually cutinized, and therefore it is through the stomata 
largely or entirely that a constant gaseous diffusion takes 
place between the air spaces of the leaf and the external 
atmosphere. The epidermis is an effective multiperforate 
septum, which means that, with a difference of gradient 
within and without, the relatively small stomatal areas 
are far more efficient in diffusion than would be suggested 
by their actual area. They are in fact sufficient to pro- 
vide for the maximum diffusion of C0 2 which may take 
place from a natural atmosphere into the plant. 

The C0 2 which enters the air chambers of the leaf is 
rapidly absorbed by the moist cell-walls within. These 
cell-walls absorb the carbon dioxid just as would any mem- 
brane moistened with water. The above capacity for 
absorption is so great that there is during photosynthesis 
practically no tension of C0 2 in the air spaces. The carbon 
dioxid in solution is presented by the cell-sap to the chloro- 
plast, and there is, of course, continuous absorption and 
migration through diffusion in solution, so long as photo- 
synthetic action proceeds. The C0 2 absorbed does not 
immigrate to any considerable distance before it is used. 
This is easily demonstrated by the fact that in small 
darkened areas no starch would be produced. It must be 
transferred, in some leaves, however, as far as the upper 
palisade laj^ers, for in these there is usually abundant 
starch-making. It is apparent that in general the sphere 
of each stoma is more or less local. The intake of carbon 
dioxid is greatest, usually, over the lower surface of the 



210 Plant Physiology 

leaf, and there the air chambers are most numerous ; but 
chlorenchyma is better developed toward the upper sur- 
face. There are, however, so many factors which influ- 
ence the structure of the leaf that the apparent inconsis- 
tency of this arrangement must be regarded as an effective 
compromise. 

The same stomatal mechanism effects, of course, a 
rapid elimination of the oxygen produced during photo- 
synthesis, after this oxygen has diffused into the air spaces 
from the moist membranes of the cells wherein it is pro- 
duced. 

114. The amount of carbon dioxid. — The amount of 
carbon dioxid in the air seems almost infinitesimal when 
we contemplate the results of its use. The air contains 
normally about .028 to .03 per cent, although this amount 
may be temporarily somewhat increased in the neighbor- 
hood of cities, or of areas where manufacturing is a chief 
industry. The limited amount of this gas suggests, 
further, the necessity of broad surfaces and the thorough 
distribution of chlorophyll. 

It has been found that the normal supply of carbon 
dioxid is often insufficient for the maximum work of the 
leaf. Under ordinary conditions, as when the plant is 
growing in strong light at a temperature of from 20 to 
25° C, and with a sufficient water-supply, a chief limiting 
factor in growth is the minimum tension of carbon dioxid. 

It has been shown experimentally that an increase in the 
amount of this gas to such extent that the air will contain 
from 1 to 10 per cent or more may be beneficial, provided 
the other factors permit a maximum activity. The results 
obtained by Godlewski and Kreusler are not entirely 



The Intake of Carbon 211 

concordant, but sufficiently so to indicate that the curve 
representing photosynthetic activity rises rapidly as the 
content of C0 2 is increased to an air content of from .1 
to 1 per cent, and subsequently the rise, if continuous, 
is slow to about 10 per cent, after which it may decline. 

The amount of C0 2 in the present atmosphere of the 
earth is sufficient for all the needs of plants throughout 
imaginable time. It must be assumed, indeed, that this 
amount will never be much more or much less than at 
present, and that, practically speaking, the forces governing 
supply and demand are ultimately somewhat regulatory ; 
although there is geological evidence that atmospheric 
C0 2 has not been constant. The result of all animal and 
plant respiration (see Respiration, p. 280) is to return to 
the air daily an enormous quantity, — an amount esti- 
mated for mankind alone to be not less than 50,000,000 
tons. The great present consumption of fuel — coal, wood, 
oil, etc. — returns to the air several billion tons every year. 
A moment's consideration of the production of coal in 
the United States alone during 1907 (400,000,000 tons 
yielding about 2 times this amount of carbon dioxid) is 
alone sufficient to indicate the immensity of the quantities 
which are involved in these exchanges. This coal repre- 
sents in part, of course, the photosynthetic activity of 
plants of the carboniferous age. In addition to these 
sources of C0 2 there is also the disintegration of rock 
carbonates. 

With rapid circulation of air the C0 2 of the atmosphere 
is evenly distributed throughout, and plants, tall and low, 
are in situations equally favorable. When, however, the 
atmosphere is quiet, there is, especially in rich ground, 



212 Plant Physiology 

rapid diffusion of C0 2 from the soil, due largely to the activ- 
ity of microorganisms of the soil. In consequence there 
may be a stratum near the soil so much richer in C0 2 as 
to be distinctly advantageous for low-lying or rosette- 
forming plants. 

115. Light the source of energy. — It has been indicated 
that an important feature of the work of chlorophyll 
is the absorption of light, or the taking over of energy. 
If a beam of sunlight is dispersed by a suitable prism, it is 
found to consist of groups of rays of different wave length, 
refrangibility, and " color " ; the beam is thus separated 
into the well-known spectrum, the visible portion of which 
presents a series of colors as follows: red, orange, yellow, 
green, blue, indigo, and violet. 

If in the path of light there is interposed a weak solution 
of chlorophyll in a suitable glass vessel, certain definite 
absorption bands appear in this spectrum. There are, 
in fact, seven of these, four in the region of red to green 
and three beyond the blue, generally rather indistinctly 
demarcated, and in strong solutions cutting out the visible 
rays beyond the blue (Fig. 56). The four bands in the 
red end of the spectrum are those of the blue-green solu- 
tion, cyanophyll ; and the most important absorption 
band is in the red, corresponding to wave lengths of 650 w 
and thereabout. 

A considerable amount of experimental work has been 
done to determine the rate of photosynthesis under the 
different monochromatic lights. Colored glass screens 
and double-walled vessels containing colored liquids have 
been much employed. Since such materials seldom afford 
pure monochromatic lights, they give only a crude idea of 



The Intake of Carbon 



213 



VJ 











214 



Plant Physiology 



the relative effects of light quality. Experiments of this 
nature are important, especially when the light employed 
is analyzed as to its energy value. It can be shown 
that bubbles of oxygen are more rapidly given off, or 
starch is more rapidly formed, under red-orange screens 
than under green or blue. Red light is therefore a chief 
source of the energy used in food-making. 

116. Efficiency of the food-making apparatus. — Pains- 
taking and brilliant investigations have been made upon 
the energy relations of leaves. The work of green plants 
is truly remarkable, and it is impressive to consider these 
organisms as the noiseless machines engaged in the manu- 
facture of all that organic material upon which life depends 
— the foremost conservators of the energy derived from 




Fig. 57. Ganong's simple light-screen and aerated box for showing the 
necessity of light in starch-making. 



The Intake of Carbon 215 

sunlight. When, however, it is asked how economic or 
efficient is this world-distributed apparatus with respect 
to the energy received, one experiences at first a keen dis- 
appointment to ascertain that the highest estimates indi- 
cate an effectiveness of only 3 per cent, and, according to 
other estimates, it may be as low as .5 per cent. Still, 
the amount of light absorbed by the leaf is considerable, 
and it is important to note the result of this. 

In diffuse light the leaf may absorb 95 per cent falling 
upon it, while in direct light only about one half is ab- 
sorbed, or reflected. In either case much of this absorp- 
tion is due to the chlorophyll bodies which have a capacity 
of from 10 to 20 times or more the amount effective in 
actual photosynthesis. The surplus energy absorbed is 
in part operative in raising the temperature of the leaf, 
which, according to Blackman, may be in direct sunlight 
from 10 to 15° C. higher than that of the surrounding air. 
This surplus, of course, induces a more intensive evapora- 
tion. Perhaps if we knew more of the physical and chem- 
ical changes involved in food-making, this efficiency would 
be unchallenged. 

n6a. Light, intensity and quality. — The relation of 
food manufacture to the intensity and quality of light is 
most complex. Under favorable conditions of tempera- 
ture the working capacity of many plants is proportional 
to the increase in light intensity, at least up to the point 
where the available C0 2 is not a limiting factor. Never- 
theless, experiments made from another standpoint indi- 
cate that with respect to photosynthesis under field condi- 
tions there are shade-loving plants — plants which seem 
to be thoroughly attuned to a maximum capacity for 



216 Plant Physiology 

food-making in weaker light. In this connection, how- 
ever, it is possible that an important factor limiting high 
production in intense light is the increased evaporation 
then resulting, which would tend to dry out trie plant 
and induce a closure of the stomata, as well as otherwise 
affect photosynthesis. The control of light intensity is 
important in crop work, as more particularly discussed 
under shading. 

116b. Temperature. — Temperature is just as impor- 
tant in food-making as in any other physiological process. 
According to Blackman the best temperature for sustained 
photosynthesis is generally about 25 to 30° C, and this 
in spite of the fact that at a temperature of about 10° the 
cell-sap may absorb and hold practically twice as much 
C0 2 as at the former temperature. The effect of higher 
temperatures upon respiration complicates the heat 
relation. It may be expected that plants adapted to 
diverse environmental conditions will not respond alike 
to heat, especially under field conditions. High tempera- 
ture is an important factor in the early maturity of wheat. 
The grain then contains relatively little starch, and the 
yield of straw is lessened. On the other hand, with ade- 
quate soil moisture, corn requires a distinctly higher tem- 
perature for abundant starch formation and maximum 
yield. 

117. Organic matter, rate of production. — A vigorous 
vine of the Concord or Niagara grape may expose to the 
light about io square meters or more of surface. Careful 
experiments with other plants indicate that the produc- 
tion (taking no account of respiration) per square meter 
of surface may be about 1 gram of organic matter per hour, 



The Intake of Carbon 217 

which has been expressed 1 gm 2 h. This gram of sugar 
involves the use of the carbon dioxid contained in 2.5 
cubic meters. At the height of the growing season we 
may count an average maximum of ten hours of work per 
day; therefore, a grape-vine of the dimensions indicated 
has the capacity of 10 X 10 = 100 grams per day, equiva- 
lent to about 400 grams (about 14 oz.) of fresh substance. 
To do this all the carbon dioxid would be taken from 250 
cubic meters of air. 

Looking at this from the standpoint of a crop per acre, 
an impressive though hazy picture may be had of the at- 
mospheric changes concerned in the making of organic 
material. A yield of 300 bushels of potatoes on an acre 
involves, including tops and roots, about 5400 pounds of 
water-free substance. Estimating as for making fruit 
sugar (2.5 cu. m. or 3.2 cu. yd. per gram) there would be 
required all the C0 2 to a height of more than one and one 
third miles over this acre, assuming no gain meanwhile. 

LABORATORY WORK 

Chloroplasls. — Study under the microscope the distribution 
of the chloroplasts in one or more types of leaves available, such 
as geranium, ivy, and tomato, using hand sections in all cases. 
Contrast one of the above with the distribution in a species of 
live-forever, purslane, or Begonia. In the best material study 
carefully under high power of the microscope the forms of the 
chlorophyll bodies and their cytological relations. In the young 
leaves of moss, Elodea, or other convenient material determine 
how multiplication of these bodies occurs. Study the form of 
the chlorophyll in desmids, procurable either from an aquarium 
or any pond containing algae. 

Light and the formation of chlorophyll. — Germinate seed of 
mustard, radish, or small grain upon moss or in small germi- 



218 Plant Physiology 

nators or pots, placing the vessels in complete darkness. After 
a few days note the color of the cotyledons and leaves, then place 
the seedlings in strong diffuse light protected by a bell glass. 
Observe the greening and note the time required to develop a 
noticeable green appearance. If convenient, some of the seed- 
lings may be put in a cold room and some at a much higher tem- 
perature, conditions of lighting being the same ; or in the same 
room the vessels may be circulated in one case with warm, and 
in the other case with cold, water. Note the effect of tempera- 
ture upon the rapidity of greening. 

Extraction of chlorophyll. — Make an alcoholic solution of 
chlorophyll from young leaves of Vicia faba, grass, small cereal, 
or radish. Put the leaves into a flask containing 95 per cent 
alcohol and heat the flask carefully over a water-bath, the latter 
being regulated to about the boiling point of alcohol (ethyl 
alcohol, 95 per cent, boils at about 78° C). When a fairly strong 
extract is obtained, pour off the solution into one or more large 
tubes and protect from the light until used ; but fresh solutions 
should be prepared for each period. Make a careful examina- 
tion of the solution (1) by reflected light and (2) in diffused or 
transmitted light. In the examination by reflected light con- 
centrate the rays upon the surface of the solution by means of 
a hand lens. 

Decomposition of chlorophyll in alcoholic solution. — Prepare 
four small test-tubes (1 inch in diameter) each with about 15 cc. 
of chlorophyll solution recently boiled and cooled. Place one 
under each of the following conditions: (1) in direct sunlight; 
(2) similar to the preceding, but with the solution covered by a 
thin layer of olive oil ; (3) in complete darkness ; (4) similar 
to (2) except in complete darkness. After an hour or two note 
any change in color; compare the tubes by holding them above 
white paper, returning each to the same condition as before, and 
observe after a lapse of about 24 hours. 

Separation of the chief pigments of chloropftyll. — Into a test- 
tube containing about 20 cc. of the fresh chlorophyll solution 
reduced in alcoholic content to about SO per cent add also 20 cc. 
benzole. Note the position taken by the benzole, then shake 



The Intake of Carbon 219 

vigorously for several minutes, cork, and let stand until the two 
areas are constant. Describe the separation phenomena. 

If a spectroscope is to be employed, as suggested in the next 
experiment, larger quantities of the materials may be used in the 
bottle, and a more complete layering effected by the addition of 
a small amount of water. In that case the two solutions are 
separated by pipetting or by the burette ; each is again washed 
with the opposite solvent, again separated, and subsequently 
employed for a determination of the absorption bands of each. 

Spectroscopic examination of chlorophyll. — If practicable, 
make, under standard conditions, a careful spectroscopic exami- 
nation of a chlorophyll solution of different strengths ; also of 
the constituents dissolved respectively in benzole and alcohol, 
resulting from the separation of the pigments above. Make a 
comparison with the living leaf, the latter preferably exhausted 
of air and injected with water, by being placed in a vessel of water 
under the exhaust or filter pump. 

Evolution of gas during photosynthesis. — Notice that when a 
mass of alga or aquatic moss, Elodea, or other water-weed is 
placed in spring water and exposed to the light, bubbles of gas 
promptly accumulate, especially from cut surfaces of the larger 
plants, and rise to the surface. No such evolution of bubbles 
takes place with control plants in the dark, although a few bubbles 
may, of course, gradually form on the walls of vessels or of sub- 
mersed plants standing for some time exposed to temperature 
changes. 

Quantity and nature of gas released in photosynthesis. — Ma- 
terials : fresh shoots of a water plant, Elodea or Cabomba ; 
a battery jar, at least 9x5 inches, filled with spring water, or 
with water into which a small amount of C0 2 has been led ; a 
funnel not more than 3 inches in diameter, with short stem ; 
a i-inch test-tube, preferably graduated, and two pieces of glass 
tubing of same diameter, — one about 5 inches long, and the other 
1 inch ; black rubber tubing suitable for attachment to the pre- 
ceding ; 2 pinch-cocks ; a ring stand with clamp ; and a netted 
wire basket 3 inches high, with cross rods at the top to support 
the funnel, all metal being paraffined. 



220 



Plant Physiology 



^ 




58. Apparatus for the determination of 
oxygen release in photosynthesis. 



Set up the experi- 
ment about as shown 
in Figure 58. Ar- 
range netted wire 
support (C), funnel 
(B), and plant in 
position. (Why so 
much distance be- 
tween (B) and walls 
of vessel (A)?) The 
water in the battery 
jar should more than 
cover the stem of the 
funnel. Arrange the 
series of tubes with 
rubber connections 
and pinch-cocks as 
shown, but fill the 
series with water and 
place the finger over 
(D) before inversion 
over the funnel. 
Support the series 
by the clamp. As 
the bubbles of gas 
come off they will be 
caught in (F), dis- 
placing water. The 
experiment should be 
set in fairly bright 
light until about two 
inches of water are 
displaced. 

When sufficient 
gas is caught, note 
the displacement, or 
mark accurately with 
a label. The gas 



The Intake of Carbon 221 

collected is to be tested as to oxygen content by means of potas- 
sium pyrogallate. 1 Unfortunately this solution would also absorb 
another gas, carbon dioxid, so it is necessary to test first for carbon 
dioxid. This is accomplished as follows : Clamp pinch-cock (G), 
remove the series of tubes from the support, and pour out water be- 
low (G), invert and fill (E) with a weak solution of potassium hy- 
drate (an absorbent of carbon dioxid) until the liquid rises in (D), 
then clamp (H) securely, open (G), and shake so that the C0 2 in 
(F) will be absorbed. Fill (D) with water, close with a finger, and 
again open the series under water to liberate any tension from 
absorption of C0 2 . If there is any change in the volume of gas 
in (F), note this, or indicate by a new label. Next, to absorb 
the oxygen, proceed in the same manner as before, except that 
fresh potassium pyrogallate is to be substituted for the potas- 
sium hydrate solution ; also the shaking, in order to absorb all 
the oxygen, should be continued for several minutes. When the 
tension is again released by inversion over water, the difference 
between the preceding and present displacement will show the 
volume of 2 caught during the experiment. Compare this with 
the volume per cent of oxygen in the air. As a check on the 
accuracy of manipulation, introduce ordinary air into the tube 
series, and then determine the volume of oxygen. (If graduated 
tube (F) is not available, accurate measurements may be made 
after the experiment is complete by filling, from a graduated 
pipette or burette, the tube to the points marked with labels 
in order to determine the volumes indicated.) 

In case the experiment is stopped for any purpose, as on 

1 The potassium pyrogallate solution now strongly recommended by 
Ganong consists of " 1 part pyrogallic acid to 5 parts caustic potash to 
30 parts of water." With this quantity of caustic potash 1 gram of the 
pyrogallate may absorb ^ gram of oxygen; but for uncontrolled experi- 
ments it is well to figure at the rate of not less than 1 gram of pyrogallic 
acid to io gram of oxygen. The solution deteriorates rapidly, even in 
diffuse light, and should be made up immediately preceding its use. 
It is preferably made by using equal parts of two solutions, each con- 
taining one of the constituents in double strength. 



222 Plant Physiology 

account of darkness, but is to be continued later, close pinch- 
cock (G) and refill with water the (DE) end of the series, before 
continuing the experiment. 

Use of the bubble-counting method to show rate of photosynthesis. 
— A general idea of the relative rate of photosynthesis under 
different conditions may be obtained by counting the number 
of bubbles of oxygen evolved in the same space of time. 

a. Method. With a rubber band attach a freshly cut sprig 
of Elodea or other water plant to a glass rod and submerse in a 
large test-tube of water at laboratory temperature, or somewhat 
above 20° C. Water from the tap generally contains sufficient 
CO;., but in long-continued experiments it may be necessary to 
lead in some C0 2 from a generator. Place the tube as above 
prepared in a wire rack in direct sunlight, and after a few minutes 
ascertain if the bubbles escape uniformly ; also the average num- 
ber given off in a unit of time, say one minute. If the bubbles 
do not come off with sufficient uniformity, try another shoot, or 
seal the cut end of the stem with wax, and then pierce a hole 
through the latter with a small needle. 

b. Light intensity. When a careful count of the bubbles 
has been made in direct sunlight, remove the tube to light suc- 
cessively weaker; note any change in the rate, and determine 
where the evolution of gas ceases. If possible, contrast the light 
intensity at this point with that of the open window, as a stand- 
ard, using an ordinary photographic actinometer. 

c. Temperature. After counting in direct light the number 
of bubbles given off when employing water at laboratory temper- 
ature, transfer the sprig promptly to water brought to a tem- 
perature of from 2 to 3° C, but otherwise similar to the preceding. 
After allowing a few minutes for adjustment, make observation 
upon the rate of 2 evolution promptly, before the sunlight has 
had an opportunity to raise the temperature appreciably; then 
warm the tube gradually to 20 or 25° C. and note the result. 

d. If time permits, determine by the bubble-counting method 
the rate of photosynthesis under blue and under orange-yellow 
screens, employing apparatus described in Chapter XVII. 

A simple test for starch. — Make a small quantity of a very 



The Intake of Carbon 223 

weak starch paste (using a piece of starch as large as a lupin 
seed in 10 cc. of water, and boil a few minutes), then add to this 
a few drops of an alcoholic solution of iodine and note the intense 
blue color. This is a common test for starch. Use the iodine 
test in determining if starch is present in leaves of nasturtium, 
geranium, or potato, which have been in bright sunlight for a 
few hours, as suggested in section 112. First extract the chlo- 
rophyll by alcohol, then stain with the iodine solution. 

Chlorophyll and photosynthesis {starch accumulation). — The 
necessity of chlorophyll in starch-making may be simply shown 
by using variegated leaves, white and green, of certain varieties 
of Coleus, or other greenhouse plants of this nature conveniently 
obtained. From a plant which has been exposed to sunlight 
several hours select a leaf, outline the white and green areas, and 
then test for starch as above suggested. Indicate the relation 
between the occurrence of starch and the areas outlined. For 
further proof place the plant in . the dark a few hours or over 
night, so that all starch is removed from the leaves, then replace 
the plant in light and determine if starch is deposited after a few 
hours, and in what areas. 

Light and photosynthesis (starch accumulation). — The obser- 
vation that leaves are depleted of starch in the dark is alone suffi- 
cient to suggest that no starch has been formed ; nevertheless, 
it is instructive to determine if starch is formed in a darkened and 
aerated area of a leaf, the remaining portion of which is exposed 
to light. Employ a Ganong aerated box or light screen (Fig. 57) 
or simpler devices similar in principle improvised for the 
purpose. After exposing the leaf for a few hours, apply the 
starch test and describe the conditions and results. 

Obtain two small potted plants, such as Fuchsia, nasturtium, 
sunflower, or jewel-weed, which shall have been determined to be 
suitable for starch formation and starch removal in the leaves. 
Place these in the dark for a few hours or over night, until a test 
indicates no starch present. Place one in strong light and the 
other in very weak light, with conditions otherwise as nearly 
the same as possible. One may be placed under a bell glass and 
the other under a bell glass covered with manila paper, or with 



224 Plant Physiology 

two or three folds of white cloth. Insert thermometers, and 
equalize the temperatures as well as possible. At intervals 
contrast the rate of starch accumulation in the two cases. 

Carbon dioxid and photosynthesis (starch accumulation) . — 
In this experiment one plant (A) is exposed co a current of air 
deprived of C0 2 , and a control (B) to similar conditions, except 
that the air is natural. Arrange the experiment preferably in 
the greenhouse or in the open, but a south window is also a pos- 
sible situation. 

Place the plants (Fuchsia is desirable) in the dark over night 
and keep them darkened until demanded. Each is covered by 
a tubulated bell glass (and with (A) is included a dish of 10 per 
cent potassium hydrate solution). Seal jar (A) to a ground 
glass or metal base, and cover both with a 2-holed rubber stopper, 
one hole serving for a connection with an aspirator or filter pump, 
and the other (in A only) connected with potassium hydrate 
wash bottles. When connections are tight, draw air through 
(A) until a baryta-water wash bottle shows no further C0 2 in 
the chamber. Then draw through both (A) and (B) a current of 
air for several hours, or as long as the experiment may be con- 
tinued in the light, and test the leaves from each plant for starch. 

References 

Blackman, F. F., and Matthei, G. L. C. Experimental Re- 
searches in Vegetable Assimilation and Respiration. Proc. 
Roy. Soc. 76 B : 402-460, 1905. 

Browx, H. T., and Escombe, F. Static Diffusion of Gases and 
Liquids in Relation to the Assimilation of Carbon [etc.]. 
Phil. Trans. Roy. Soc. 193 B : 223-291, 1900. 

Browx, H. T., and Wilson, W. E. On the Thermal Emissivity 
of a Green Leaf in Still and Moving Air. Proc. Roy. Soc. 
76 B : 122-137, 1905. 

Brown, H. T., and Escombe, F. Physiological Processes of 
Green Leaves. Proc. Roy. Soc. 76 B : 29-111, 1905. 

Czapek, F. Die Ernahrungsphysiologie der Pflanzen seit 1896. 
Progressus Rei Botanicas. 1 : 419-532 [468-477]. 



The Intake of Carbon 225 

Hansen, A. Die Ernahrung der Pflanzen. 299 pp., 79 figs, 1898. 
Kreusler, U. Ueber die Methode zur Beobachtung der 

Assimilation und Athmung der Pflanzen [etc.]. Land- 

wirthsch. Jahrb. 14:913-965, 1885; 16:711-755, 1887; 

17 : 161-175, 1888 ; 19 : 649-668, 1890. 
Marchlewski, L. Die Chemie der Chlorophyll. 187 pp., 

7 ph., 5 figs., 1909. 
Schunck, C. A. The Xanthophyll Group of Yellow Coloring 

Matters. Proc. Roy. Soc. 72 : 165-176, 2 pis., 1903. 
Senn, G. Die Gestalts- und Lageveranderung der Pflanzen- 

Chromatophoren. 397 pp., figs., 9 pis., 1908. 
Timiriazeff, C. The Cosmical Function of the Green Plant. 

Proc. Roy. Soc. 72 : 424-461, 1903. 

Texts. Barnes, Ganong, Jost, Pfeffer. 



CHAPTER X 
THE RELATION TO NITROGEN 

The significance of the nitrogen content of soils has been 
recognized deservedly as so important in crop production 
that a stupendous number of investigations has been 
directed toward securing the facts regarding the diverse 
relations of this nutrient. Many of these investigations 
have yielded data of surprising interest respecting the use 
of nitrogen by higher plants and by microorganisms as 
well. Furthermore, the results have been sufficient to 
demonstrate most clearly the intimate relations existing 
between soil bacteria and cultivated crops as regards the 
nitrogen supply. With phosphoric acid and potash it 
constitutes the trio of nutrients which experience has de- 
manded as usually the most important fertilizers for crop 
production. 

118. Combined nitrogen. — The water-culture experi- 
ments (sections 78-80) have demonstrated in a manner 
sufficiently convincing that the nutrient solutions for 
higher plants must contain nitrogen. As soon as the 
supply of this clement in the seed is fairly exhausted the 
addition of combined nitrogen is required. About the 
middle of the last century Boussingault showed conclu- 
sively that the N 2 of the air, unlike the CO.,. is not directly 
serviceable as a source from which nitrogenous foods 
226 



The Relation to Nitrogen 227 

may be made by the green plant. The air contains about 
78 per cent of " free " nitrogen, but this vast source is 
wholly inert naturally except, as later indicated, either 
through the intermediary of certain microorganisms or 
by means of electric discharges, whereby this free nitrogen 
is combined. Nevertheless, since the original rocks seem 
to contain no nitrogen, the air is the original source of all 
that at present found in arable soils — constituting often 
from .1 to .3 per cent of the dry weight of the soil. 

119. The nitrogen content of plants. — Nitrogen 
enters into a variety of organic compounds among which 
the proteins are of the greatest importance, for these in 
turn are apparently the main constituents of protoplasm, 
whether living or dead. Some other compounds, occur- 
ring in plants, which contain nitrogen are various amino 
and amido acids, certain alkaloids, and also nitrates. 
The protein content is greatest in seeds, storage organs, 
and meristematic tissues. In beans and other legumi- 
nous plants it may amount to 25 per cent of the dry weight, 
while in wheat straw it constitutes only about 3.5 per cent. 

120. Synthesis of nitrogenous bodies. — Animals ob- 
tain nitrogen, for the most part, as protein foods, furnished, 
of course, by the bodies or products of other animals or 
plants. On the contrary, the rule is that green plants and 
many fungi and bacteria are able ultimately to construct 
amido compounds, proteins, and other nitrogenous bodies 
from certain of the raw materials; that is, from some 
of the mineral nutrients and photosynthates (carbohy- 
drates) . 

Proteins represent an immense group of compounds 
(sections 147-148) with a relatively enormous molecule. 



228 Plant Physiology 

They must contain carbon, hydrogen, oxygen, and nitro- 
gen; many contain sulfur, certain forms phosphorus, 
and others apparently mineral bases, the latter either 
combined or as ash. Proteins may be regarded as per- 
fect foods for protoplasm. 

Lack of knowledge respecting the proteins renders 
impossible a clear picture of their synthesis, although 
much may be inferred from the decomposition products, 
which have been extensively studied. Speaking generally, 
the amides (containing the group NH 2 ) seem to represent 
products intermediate between certain of the raw mate- 
rials (organic acids or carbohydrates and nitrates) and 
the proteins. Glycin, for a simple example, is an amido- 
acetic acid [CH 2 (NH 2 ) ■ COOH] in which the amide group 
replaces one H of the acid. It may be assumed that pro- 
tein is constructed from amido compounds, especially 
from those derived from carbohydrates, some of which 
may be further modified by the incorporation of sulfur 
from sulfates, and others by the introduction of phos- 
phorus from phosphates. 

121. Soil nitrogen. — Some data regarding the nitro- 
gen relations of higher plants have been exhibited in con- 
nection with the discussion of mineral nutrients. On 
account of the unusual importance of the nitrogen supply 
in any permanent system of agriculture, special attention 
should be given to the interesting transformations of ni- 
trogenous materials in the soil, and likewise the building 
up of nitrogenous bodies within the plant require some 
consideration. 

It is now a matter of common knowledge that nitro- 
gen exists in the ordinary arable soils in a variety of com- 



The Relation to Nitrogen 229 

binations. Aside from the N2 of the air there may be 
undecomposed organic matter, containing most of the com- 
pounds of the plants and animals which it represents. 
There is also the converted organic matter, that resulting 
from decay, — commonly designated humus, — which may 
consist, in fact, of a variety of substances. Finally there 
is the inorganic nitrogen, including nitrates, often a small 
proportion of nitrites, and compounds of ammonia. The 
total nitrogen content of the soil is therefore most diverse, 
but in productive agricultural soils there is invariably 
a considerable nitrate content during the growing season. 
It is probable that under exceptional conditions higher 
plants may use to a certain extent organic nitrogen in the 
form of amido compounds, but, practically speaking, it 
seems certain that such organic bodies are not absorbed 
or utilized in sufficient quantity to make this question one 
of importance. 

122. Nitrites. — The nitrites are commonly injurious, 
and the presence of these in any quantity is a sure indica- 
tion of unfavorable conditions. As will be shown sub- 
sequently, they occur as temporary products during the 
oxidation of ammonia to nitrates, but may be looked upon 
under favorable conditions as merely transitory. 

123. Nitrates. — By means of water cultures it is 
relatively a simple matter to determine that nitrates are 
the most favorable source of nitrogen in water or sand 
cultures under normal conditions, and more especially 
under sterile conditions. It is certain that such com- 
pounds are usually absorbed by the plant unchanged. 
Nitrates of the various nontoxic bases are therefore val- 
uable direct fertilizers, and the data furnished by the 



230 Plant Physiology 

extensive fertility experiments throughout the world lead 
to the conclusion that the maintenance of an adequate 
nitrate supply in the soil, or of conditions leading to the 
transformation of nitrogenous matter into nitrates, is an 
important principle in production. 

The nitrates are readily soluble, and while this character 
enhances the rapid action of such substances as fertilizers, 
it is at the same time a quality making possible constant 
loss through percolation and leaching. It is evident, 
therefore, that unless the nitrate supply is maintained by 
natural or artificial means, exhaustion of this requisite 
element would sooner or later occur. Fortunately, there 
are both natural means as well as conditions of cropping 
which may suffice to maintain and to increase the nitrogen 
content, as developed later. 

124. Compounds of ammonium. — Prior to the latter 
quarter of the last century the prevailing view was to the 




Fig. 59. Fertilization of grass land 

and acid phosphate; also, from Left to right, no nitrogen, I nitrogen 

ration, and full nitrogen ration. [Photograph from the Rhode Island 
Exp. Sta.] 

effect that compounds of ammonium (such as the sulfate, 
chlorid, and nitrate) should be considered the important 
sources of nitrogen for plants, and the weight of Liebeg's 
opinion was upon it. At that time the cycle of changes 
involving nitrogen was incompletely known. Unques- 



The Relation to Nitrogen 231 

tionably the addition of ammonium salts to the soil gave 
increased yields, and the inference was that they were 
directly beneficial; that is, that they were absorbed as 
salts of ammonium. 

Subsequently, when it was determined that, as a result 
of nitrification, ammonium compounds may be oxidized, 
ultimately to nitrates, the dominant view was to regard 
nitrates as practically the only source of nitrogen for crops. 
This is still held by many, but the relatively recent exper- 
iments of Maze, 1 Hutchinson and Miller, 2 and others 
seem to indicate that salts of ammonium are directly 
absorbed. In some cases nitrogen in the latter form 
afforded growth equal to that where nitrates were em- 
ployed, and the nitrogen content of peas is reported greater 
when ammonia is the source of nitrogen. Nevertheless, 
the salts of ammonium are more toxic than nitrates, this 
toxicity exhibiting itself at the lower concentrations merely 
in depressing the growth of roots. 

125. The sources of soil nitrates and ammonia. — 
Briefly stated, the supply of nitrates and ammonium com- 
pounds in the soil annually removed by crops, by leach- 
ing, and through denitrification (section 130) are or may 
be renewed by the following means, most of which are 
subsequently discussed : — 

(1) By the decomposition or decay of organic matter, 
accomplished by microorganisms. 

(2) By means of the bacteria producing and inhabiting 
the root-tubercles of leguminous plants, which bacteria 
possess the power to " fix " atmospheric nitrogen. 

1 Maze, P., Ann. de l'inst. Pasteur, 14 : 23-45, 1898. 

2 Hutchinson, H. B., and Miller, N. H. J., Journ. Agl. Sci., 3 : 179- 
194, 1909. 



232 Plant Physiology 

(3) By the action of certain soil bacteria and fungi 
which are also able to utilize atmospheric nitrogen. 

(4) By the ammonia returned to the soil as a result of 
rainfall; but since this, in general, is that which escapes 
from the soil into the air, it is negligible. 

(5) As a result of electrical discharges nitrous and nitric 
acids may be produced in the air and through rains brought 
to the soil, but this amount is relatively inconsiderable, 
consisting, under the most favorable conditions (in the 
moist tropics), of about five pounds per acre annually. 

126. Ammonification. — The remains of plants and 
animals are " returned " to the soil through processes of 
decay and putrefaction brought about largely by means 
of fungi and bacteria. Decay is a relative term usually 
implying decomposition without the production of mal- 
odorous compounds, and commonly taking place with 
access of oxygen. In putrefaction ill-smelling compounds 
result, usually from the decomposition of nitrogenous 
substances, taking place, as a rule, with poor oxygenation. 

A result of both of the above processes is that nitrog- 
enous compounds are broken down into ammonia, carbon 
dioxid, and other products. This reduction to ammonia 
constitutes what is known as ammonification. Under 
favorable conditions a large part of the ammonia is held 
in the soil by entering into combinations with the soil 
bases. During this decomposition many substances more 
or less injurious may be at least temporarily set free in 
the soil. 

Numerous species of bacteria and fungi affect decom- 
position. Bacteria are particularly important in arable 
soils, especially such species as Bacillus mycoides and B. 



The Relation to Nitrogen 233 

vulgaris, while many common molds, punks, and mush- 
rooms are among the various fungi inducing decay in the 
forest. 

127. Nitrification. — Ammonification is the comple- 
tion of the first stage in the cycle of changes whereby ni- 
trogenous matter may be converted to nitrates, the suc- 
ceeding transformations being as follows : — 

(1) Oxidation of the salts of ammonium into nitrites. 

(2) Oxidation of nitrites into nitrates. 

The production of nitrates (ultimately) from organic 
matter has been long known and practically carried out 
by means of the niter beds so much employed a generation 
or two ago. A process wholly similar in nature has given 
rise to the natural deposits of niter and is constantly at 
work in the best arable soils to develop nitrates. 

128. Nitrifying organisms. — In 1877 it was first 
determined (Schloesing and Muntz) that nitrification is 
effected by bacterial agencies, and Winogradski, Waring- 
ton, Godlewski, and others have laid bare many impor- 
tant features affecting the action of the organisms involved. 
These bacteria are widely distributed in soils and in drain- 
age or run-off waters. 

The organisms oxidizing ammonia to nitrites (nitrite 
organisms) are small, oval, motile cells generally included 
in the genus Nitrosomonas, while those oxidizing nitrites 
(nitrate organisms) are considered nonmotile and included 
in the genus Nitrobacter. 

These two types of bacteria are commonly associated 
in the soil, and all forms seem to exhibit the peculiar 
physiological quality of being able to make their own 
carbohydrate food from C0 2 and water. Unlike green 



234 



Plant Physiology 



plants, they accomplish this in the absence of light (chemo- 
synthetically). 

In cultivated soils they are most abundant below the 
surface mulch, and down to the limits of frequent culture. 
The following table shows the distribution of nitrifying 
bacteria and nitrates at different depths in a Dakota 
soil 1 : — 



Soil Depth 


Colonies of nitrifying 
Bacteria 


Xitrates, Pounds 
per Acre 


3 in. 


2300 


415.9 


6 in. 


2300 


415.9 


12 in. 


600 


234.3 


18 in. 


200 


196.6 


24 in. 


10 


430.9 


36 in. 





674.9 


48 in. 





316.3 


60 in. 





395.3 


72 in. 





247.0 


84 in. 





293.9 



129. Conditions favoring nitrification. — The general 
conditions favorable for nitrification in soils are good 
aeration (oxygen and carbon dioxid), a medium water- 
content, a soil temperature not to exceed 40° C, the 
presence of a basic compound such as calcium carbonate, 
and the absence of much soluble organic matter and free 
ammonia. In general these conditions are those of good 
sanitation and tilth, a judicious application of lime, and 
such a rotation of crops as will produce and maintain the 
best of soil conditions. The advantage of paying the 

i Ladd, E. F., North Dakota Agl. Exp. Sta., Bui. 47 : pp. 685-704. 



The Relation to Nitrogen 235 

closest attention to those conditions resulting in the high- 
est nitrification is obvious. 

As a rule nitrate formation in the soil begins rapidly in 
the spring and with most crops a maximum is reached 
during the first half of the growing season ; subsequently 
there is a fall in the nitrate content which may approach 
a minimum in the late fall, or with the maturity of the crop. 
Recent studies upon the relation of crops to the nitrate 
content have developed a number of interesting views which, 
however, may not be discussed in this place. 

130. Denitrification. — Almost the counterpart of nitri- 
fication is the process exhibited by many micro-organisms 
of reducing nitrates and nitrites, known as denitrification. 
From an agricultural standpoint the most serious case is 
that of the reduction of nitrates and nitrites with the forma- 
tion of free nitrogen, and the consequent loss to the soil 
of combined nitrogen. In many cases, however, the reduc- 
tion is not carried so far as to form free nitrogen. 

A large number of organisms are able to accomplish 
nitrate reduction, both bacteria and fungi ; but the condi- 
tions under which denitrification occurs are not usually 
those developed under the best agricultural practices. 

Aside from the presence of necessary nitrates this reduc- 
tion requires the presence of considerable soluble organic 
matter and poor aeration. Many of the organisms which 
induce denitrification are aerobic forms which in the pres- 
ence of sufficient oxygen show no tendency toward nitrate 
reduction. It is apparent that saturation of the soil with 
water after heavy manuring may actually result in nitro- 
gen loss and frequently also in the production of in- 
jurious compounds in the soil. 



236 Plant Physiology 

131. Nitrogen fixation. — Since by nitrification (in- 
cluding ammonification) nitrogenous bodies are merely 
transformed into inorganic nitrogen, this does not increase 
the total nitrogen of the soil. Moreover, the nitrogen 
brought to the soil as a result of electrical discharges is a 
small amount. It is then apparent that the loss of com- 
bined nitrogen over the surface of the earth through the 
washing away of sewage, the leaching of soils, and the 
liberation of free nitrogen in denitrification would mean 
in time a nitrogen famine. There is, however, a more than 
compensating process of nitrogen fixation. 

132. Organisms which fix free nitrogen. — Since the 
classical researches of Hellriegel and Wilfarth, there has 
accumulated a vast array of facts and observations with 
respect to the fixation of free nitrogen by micro-organisms. 1 
The role played by the bacteria of the leguminous tuber- 
cles was the first to be clearly demonstrated, but the im- 
portance of certain saprophytic soil bacteria in the process 
of nitrogen accumulation was fully recognized a short 
time later. Strikingly little lias been said in agricultural 
publications regarding the r61e which may be played by 
fungi in this process. Nevertheless, as a result of a series 
of observations and experiments, it is now commonly 
held that certain fungi are likewise important in fixation, 
and this view is regarded in the succeeding discussion, 
although there is some doubt respecting the real impor- 
tance of the fungi in this connection. 

133. Bacteria of leguminous tubercles. — In recent 

1 The evidence now commonly accepted is to the effect that certain 
bacteria and fungi alone among all organisms possess the capacity for 
nitrogen fixation. 



The Relation to Nitrogen 237 




Fig. 60. Roots of vetch with clusters of fan-shaped tubercles. 



238 Plant Physiology 

times no plant structures have, perhaps, attracted more 
attention than the nodules, or tubercles, of the leguminous 




% 

Fig. 61. Infection thread and abnormal tissue in a tubercle developing 
upon the root of Vicia sativa. At the right, infection thread in root- 
hair. [After Geo. F. Atkinson.] 

plants. These structures are abnormal growths resulting 
from the attacks of parasitic bacteria, Figures 60 and 61. 



The Relation to Nitrogen 239 

Parasites generally make little or no return to the hosts 
in which or upon which they live. These nodule bacteria, 
Pseudomonas radicicola, are exceptions to this rule. The 
tubercles are, in fact, root colonies of the micro-organisms. 
The bacteria get their carbon, minerals, and water from 
the host, yet ultimately they give to the host in return 
combined nitrogen which has been acquired by the fixa- 
tion of the free nitrogen of the air. 

In the earliest days of historic agriculture, it was known 
that leguminous plants benefit the land for succeeding 
crops. The methods by which benefit results were, of 
course, unknown. When it was ascertained that the chief 
benefit is concerned with the accumulation of nitrogen, 
it was assumed that these legumes and other plants might 
themselves be able to assimilate atmospheric nitrogen. 
BoussingauhVs experiments clearly demonstrated the incor- 
rectness of this view. His work was convincing, and 
finally attention was directed to the tubercles of legumi- 
nous plants as the cause of nitrogen accumulation. 

Hellriegel and Wilfarth demonstrated that on sterile 
soil no tubercles are present and no nitrogen is fixed. 
The complete chemical and biological studies which sub- 
sequently followed have led to a full confirmation of the 
work of Hellriegel and his associate. It is now a simple 
matter to determine that leguminous plants growing in 
the absence of the bacteria are wholly dependent upon the 
soil supply of combined nitrogen, whereas, in the presence 
of the proper bacteria, such plants are able to reach normal 
development with a deficient soil supply of nitrogen, and 
even to give to such deficient soil, through root decay, an 
increased nitrogen content. 




240 Plant Physiology 

The bacteria producing the tubercles seem to have 
acquired at least racial specialization, so that no one form 

of the organism will 
infect all leguminous 
plants. The introduc- 
tion of a particular 
legume into a region in 
which that plant (or a 
closely related species) 
has not been grown 
may necessitate, for 
best results, " inocula- 
tion " of the soil or of 
the seed employed. 

Fig. 62. Bacterioids from legume tu- Formerly the Organ - 

bereles : Melilotus alba (1), Medicago . . - - 

satica {2, 3, and J), and Vicia villosa (4). lsm was introduced 

[After Harrison and Barlow.] by importing soil 

from a locality in which the legumes had been grown. 
This method has many disadvantages, and at the present 
time a very thorough test is being made of the practicabil- 
ity of employing pure cultures of the organism desired. 
Good results have been secured with pure cultures in many 
cases, but in some particulars the method is still in the 
experimental stage. 

134. Certain saprophytic soil bacteria. — Evidence 
that saprophytic soil bacteria are able to fix free nitrogen 
was afforded when Berthelot found in 1885 that bare soil 
with its normal population of micro-organisms may con- 
siderably increase in nitrogen content over and above 
that added through rainfall. At the same time, no increase 
occurred upon eliminating micro-organisms by steaming. 



The Relation to Nitrogen 



241 




Fig. 63. Crimson clover inoculated (background) arid uninoculated 
(foreground) . [Photograph by J. F. Duggar.] 
R 



242 Plant Physiology 

Ten years later Winogradski isolated Clostridium 
Pasteurianum, an organism which proved to be capable 
of fixing free nitrogen when grown in the absence of air 
(oxygen), also similarly capacitated in the presence of air 
when associated with other bacteria utilizing free oxygen. 
Since that time much work has been done. Several other 
species of soil bacteria having the power of fixation have 
been isolated, these latter being included under the genus 
Azotobacter. They are common and important in arable 
soils containing a relatively small amount of combined 
nitrogen; moreover, their activity is enhanced by the 
presence of considerable lime and by general fertility as 
regards the other mineral nutrients. From the majority 
of experiments thus far reported it does not appear that 
the addition to the soil of cultures of these organisms has 
occasioned increased nitrogen fixation. 

135. Fungi. — The investigation of nitrogen fixation 
by fungi has yielded many data of interest, although there 
is some conflicting evidence. Fixation of nitrogen by 
fungi was reported as early as 1862, but the more impor- 
tant work has been done since 1895. Among several 
saprophytic and parasitic species employed, Saida secured 
maximum fixation with cultures of Phoma Betce, a fungus 
normally parasitic upon sugar-beets. 

Several observers have reported fixation for a few of the 
common molds of soils and decaying vegetation, including 
Aspergillus niger and Penicillium glaucum. In all cases 
the organisms were grown in pure cultures, either in the 
absence of combined nitrogen, or in the presence of very 
small quantities of such compounds. In no case does the 
amount of nitrogen fixed amount to more than a few 
milligrams. 



The Relation to Nitrogen 



243 



136. Mycorhizal fungi. — Ternetz has isolated and 
tested a fungus from the roots of certain heaths. The 
association of root and fungus is known as mycorhiza. 
This fungus proved to be a species of Phoma, and several 
strains of it were found to show a high capacity for nitro- 
gen fixation. In fact, while the total amount of nitrogen 
fixed in a given period of time is relatively small, the fixa- 





Fig. 64. 



Mycorhiza of the orchid Corallorhiza, also cells showing the 
hyphse. [After M. B. Thomas.] 



tion per gram of dextrose used by the fungus indicates that 
in pure cultures fixation is more economic than with any 
other organisms yet investigated. The following table 
affords a comparison of the efficiency of several strains of 
this fungus as compared with certain soil bacteria : — 

The Phoma discussed above develops an endophytic 
mycorhiza, penetrating the cells. This endophytic form 



244 



Plant Physiology 



Organism 


Time in 
Days 


Dextrose 

FUR- 
NISHED, 

GRAMS 


Nitro- 
gen Fixa- 
tion, MIL- 
LIGRAMS 


Nitrogen 
Fix. per 
Gram Dex- 
trose, MILLI- 
GRAMS 


Clostridium Pasteurianum . . 


20 


40 


53.6 


1.34 


Clostridium Pasteurianum 






20 


20 


24.4 


1.22 


Clostridium Amerieanum 






30 


1.25 


4.6 


3.7 


Clostridium Amerieanum 






30 


5 


8.2 


3.01 


Azotobacter chroococcum 






35 


5 


42.7 


8.56 


Azotobacter chroococcum 






. 35 


12 


127.9 


10.66 


Phoma radicis Oxycocci 






28 


7 


15.3 


18.08 


Phoma radicis Andromeda? 




28 


7 


7.3 


10.92 


Phoma radicis Vaccinii 




28 


7 


15.7 


22.41 



of mycorhiza occurs also in several orchids and in a few 
other plants. Many common forest trees, such as beech 
and pine, likewise exhibit mycorhiza. In the latter case 
the fungus invests the root with a mycelial weft, the threads 
merely coming in close contact with the cells (ectophytic). 
These fungi are believed to be not only of importance to 
the tree as absorbing organs for water and nutrient salt-, 
but possibly in the fixation of nitrogen. Nevertheless, 
this point has not been established. 

137. General sources of supply of nitrogen. — From 
the data presented it is apparent that the nitrogen problem 
in plant production is one of peculiar interest and diver- 
sity. Commercial sources of nitrogen for plant production 
may be natural supplies of nitrates, compounds of am- 
monium (chiefly the sulfate, as a by-product of coke and 
gas making), waste and prepared animal products, and 
green-manure crops (especially legumes). In addition, 
good conditions for fixation by bacteria and fungi and 



The Relation to Nitrogen 245 

rotation with legumes are supplementary means of nitro- 
gen restoration. Finally, electric fixation is a source of 
supply. 

138. Electric fixation of nitrogen. — In recent years 
several methods have been devised for the oxidation of 
atmospheric nitrogen. These methods involve the use of 
cheap power, since high-power electric currents are neces- 
sary. Those which are now important in the production 
of materials commercially valuable as fertilizers require 
also a cheap source of lime. The two methods referred 
to are known respectively as the Birkeland-Eyde process 
and the calcium carbide process. By the former a basic 
lime nitrate is produced, and by the latter a lime-nitrogen 
consisting of calcium cyanamid and calcium cyanide. The 
first mentioned product, which is much employed in north- 
ern Germany, is a direct fertilizer, whereas the latter must 
first undergo decomposition in the soil. The cyanamid 
is more easily handled, the basic nitrate being strongly 
hydroscopic. 

LABORATORY WORK 

Ammonification. 1 — The decomposition of protein with for- 
mation of ammonia may be demonstrated by the action of certain 
bacteria upon egg albumen. Prepare a solution containing about 
2 grams of egg albumen in 50 cc. of water, and to prevent coagu- 
lation add 50 cc. of .05 ferrous sulfate. Pour about 10 cc. of 
the solution into each of several test-tubes, sterilize for 1 hour 
at 100° C, cool, and inoculate some of these with a pure culture 
of Bacillus mycoides, or some other organism reported to possess 

1 For more experiments upon ammonification, nitrification, and 
related phenomena the student should consult especially Percival's 
"Agricultural Bacteriology," 1910. 



246 Plant Physiology 

the power of ammonifieation. Place the cultures at a tempera- 
ture of 28 to 30° C, and in two weeks test the inoculated and 
uninoculated tubes for ammonia with Nessler's solution. 

The Nessler solution is prepared by dissolving 2 grams of 
potassium iodide in 5 cc. of hot water, while warm add mercuric 
iodide to excess of solution, cool, dilute with water to 25 cc, 
shake, settle, filter, and then dilute the filtrate to 50 cc. with a 
concentrated solution of caustic potash. This solution assumes 
a yellow color when there is added to it a few drops of a solution 
containing ammonia. 

Nitrification. — A crude but simple demonstration of nitri- 
fication phenomena, usually successfully carried out, may be 
made with impure cultures as follows : — 
Prepare a solution containing — 

Ammonium sulfate 5 gram 

Dipotassium phosphate 5 gram 

Sodium chlorid 2 gram 

Magnesium sulfate 2 gram 

Ferrous sulfate 05 gram 

Water 200 cc. 

Weigh out .5 gram of basic magnesium carbonate into each of 
four small Erlenmeyer flasks, add to each 50 cc. of the above 
salt solution, plug with cotton, and sterilize. When cool inocu- 
late three of the flasks with a small quantity (about .1 gram) 
of garden loam taken 5 or 6 inches below the surface of the soil. 
Save the fourth flask as a control. Place all at a temperature 
of 28 to 30° C. Once a week remove from the inoculated flasks 
from 3 to 5 cc. of solution and make the following tests : — 

1. For ammonia. Employ Nessler's solution. 

2. For nitrites. Acidulate with sulfuric acid about 2 cc. of 
the solution, add a few drops of potassium iodide and starch 
paste. If nitrates are present the starch is colored blue from the 
reduced iodine. 

3. For nitrates. When from (2) it is evident that nitrites 
are no longer present, dissolve a crystal of diphenylamine in 
about 1 cc. of sulfuric acid in an evaporating dish. The addi- 



The Relation to Nitrogen 247 

tion of a drop or two of the culture solution will give in the 
presence of nitrates (no nitrites being present to give the same 
reaction) a blue-violet color. At the close of the experiment 
test thoroughly the control in order to determine if the ammo- 
nium salt has remained unchanged. 

Denitrification. — To about 100 cc. of prepared nutrient 
bouillon (consult any bacteriology) add 3 grams of sodium ni- 
trate, and pour about 10 cc. of the solution into each of several 
test-tubes. Inoculate the tubes from a pure culture of some 
denitrifying organism, such as Bacillus denitrificans, or each 
tube with about .5 gram of fresh cow manure. Place the tubes 
at a temperature of from 30 to 35° C. Note all the changes in 
appearance of the culture and test occasionally for nitrates as in 
the preceding experiment. Discuss the results. 

Root tubercles. — Study the root tubercles of several legumi- 
nous plants, such as vetch, red clover, and alfalfa, with special 
reference to the form and distribution of the nodules. 

Examine prepared slides to determine the distribution of the 
organism within the tissues. If prepared slides are not at hand, 
make sections, stain in gentian violet, counterstain with orange 
G., locate the band-like colonies of the organism, and note the 
general conditions of the tissue modifications. 

Crush a bit of tissue from the inner portion of the nodule on a 
clean cover glass, dry, and stain several hours in dilute gentian 
violet ; then rinse the cover glass, dry, and mount in balsam. 
This affords a satisfactory preparation for an examination of the 
organism (Fig. 62). 

If facilities are at hand, determine the necessity of the bacteria 
for tubercle production and contrast the growth of certain leg- 
umes in inoculated and uninoculated soil. The experiment can 
only be relative unless much care is taken with sterilization 
precautions. The materials needed are six pots of fairly poor 
soil, a pure culture of the root tubercle bacteria, seed of the legume 
host plant, and a .2 per cent solution of formaldehyde. Steam 
the pots of soil 2 hours, disinfect the seed by soaking 1 hour in 
the formaldehyde solution, then sow a few seed in each pot. 
Inoculate three of the pots with the pure culture and leave three 



248 Plant Physiology 

as controls. Place the pots in the greenhouse upon fresh cinders, 
separating the two lots and protecting the pots as well as practi- 
cable from contamination by dust or soil from other pots or beds. 
Water with distilled water only. After the plants have grown 
sufficiently, compare the two lots, as indicated, for nodule for- 
mation and amount of growth. 

References 

Atkinson, G. F. Contribution to the Biology of the Organism 
causing Leguminous Tubercles. Bot. Gaz. 18: 157-166; 
226-237 ; 257-266, 1893, 4 ph. 

Atwater, W. O. On the Acquisition of Atmospheric Nitrogen 
by Plants. Amor. Chem. .\r. 6:365-388, 1885. 

Beyerinck, M. W. Die Bakterien der Papilionaceen-Knoll- 
chen. Bot, Zeit, 46:723-735; 741-750; 757-771; 
781-790; 797-804, 1 pi., 1888. 

Harrison, F. C, and Barlow, B. The Xodule Organism of the 
Leguminosae. Its Isolation, Cultivation, Identification, and 
Commercial Application. Cent. f. Bakt. II Abt. 19: 
264-272; 426-441. 9 ph., 1907. 

Hellriegel, H., and Wilfarth, H. Untersuchungen iiber die 
Stickstoffnahrung der Gramineen und Leguminosen. Zeit. 
des Vereins fur Riibenzucker Industrie. Beilageheft, 234 pp., 
1888. [Abs. in Biedermann's Central. 18:17'.) 185, 1889.] 

King, F. H., and Whitson, A. R. Development and Distribu- 
tion of Nitrates in Cultivated Soils. Wisconsin Agl. Exp. 
Sta. Bui. 93: 39 pp., 6 figs., 1902. 

Lipman, J. G. Report of the Soil Chemist and Bacteriologist. 
N. J. Agl. Exp. Station Report for (1907) : 139-204 pp., 
3 ph., 1908. 

Lurz, L. Les Microorganismes, Fixateurs d'azote Disserta- 
tion. (1904) : 187 pp., 19 figs., 1904. 

Peirce, J. G. The Root-tubercles of Bur Clover and of Some 
Other Leguminous Plants. Proc. of the California Acad, 
of Sciences. (3d Ser.) 2 : 295-328, 1 pi, 1902. 

Prazmowski, Adam. Die Wurzelknollehen der Erbse. Landw. 



The Relation to Nitrogen 249 

Versuchsstation. 37:161-238, 2 pis., 1890; 38:5-62, 

1891. 
Rossi, Gino de'. Ueber die Mikroorganismen, welche die 

Wurzelknollchen der Leguminosen erzeugen. Cent. f. 

Bakt. II Abt. 18:289-314; 481-489, 2 ph., 1907. 
Saida, K. Ueber Assimilation freien Stickstoffes durch Schim- 

melpilze. Ber. d. deut. bot. Ges. 19: (107)-(115), 1901. 
Ternetz, Charlotte. Uber die Assimilation des atmosphar- 

ischen Stickstoffes durch Pilze. Jahrb. f. wiss. Bot. 44 : 

353-408, 1907. 
Voorhees, E. B., and Lipman, J. G. A Review of Investiga- 
tions in Soil Bacteriology. Office of Expt. Stations, U. S. 

Dept. of Agl. Bui. 194 : 108 pp., 1907. 
Ward, H. M. On the Tubercular Swellings on the Roots of 

Vicia Faba. Phil. Trans. Roy. Soc. Lond. 178 : 539-562, 

2 ph., 1887. 
Woronin, M. Uber die bei der Schwarzerle (Alnus glutinosa) 

und der gewohnlichen Gartenlupine (Lupinus mutabilis) 

auftretenden Wurzelausschwellungen. Mem. de l'Acad. 

Imp. de St. Petersbourg. [7] 10 : 1-13 pp., 2 ph., 1866. 

Texts. Barnes, Jost, MacDougal, Pfeffer, Peirce. 



CHAPTER XI 

PROD UC TS OF MET A BOLISM; DIGES TIO N 
A ND TRANSLOCA TIO N 

From the discussion of the general relations of the green 
plant to carbon and nitrogen it has been developed that 
a variety of organic products are characteristic of the plant 
cell and plant body. Beginning, in the typical case, with 
those which may be regarded as the products of photo- 
synthesis (photosynthates), on the one hand, and with 
the elementary organic substances containing nitrogen. 
on the other, there may be built up in various ways diverse 
series of organic compounds, constituting the plant body 
and including, of course, the protoplasm and the cell 
walls. This shall not be taken, however, to indicate that 
there is a normal sequence of products, or a continuous 
building up, for, as will be shown subsequently, the build- 
ing up may be interrupted at any point and breaking down 
may occur. The food products are then utilized in a 
manner dependent upon the specific nature of the sub- 
stances and upon the chemical requirements of the cell. 
Pfeffer has distinguished plastic and aplastic substances. 
The former term is used by him to include substances 
which may be used as food, which may be mobilized and 
utilized in metabolism; while aplastic substances include 
the solid and permanent constituents of the cell, — such 
as the cell wall, — and certain by-products or waste ma- 
250 



Metabolism ; Digestion and Translocation 251 

terials. It is difficult, of course, to draw any line between 
these two types of substances. 

139. Metabolism. — All of those chemical changes 
which take place within the body incident to growth and 
development are commonly included under the term " me- 
tabolism." These changes may be constructive (or ana- 
bolic) and destructive (or catabolic). Some brief indica- 
tions have been given respecting the building up of a few of 
the more important organic compounds, and it is necessary 
to include now a somewhat comprehensive view of the 
general relations of a few of the foods and by-products 
and some characteristics of these materials. 

140. Temporary foods, storage products, and perma- 
nent structures. — It would seem that many substances 
produced within the cell are temporary, that is, they may 
be labile compounds readily used in the metabolism of 
the active cell. If formaldehyde is a first product of photo- 
synthesis, it is necessarily one of this nature. Naturally 
the transient compounds are the lesser known as plant 
constituents ; but it seems certain that many simple car- 
bohydrates, fatty acids, amides, and the like are distinctly 
temporary. Nevertheless, a substance which is temporary 
in one plant may be accumulated in another. 

Whenever the food manufactured is in excess of that 
used, it accumulates, and may be regarded as a storage 
product. The chlorenchyma of higher plants is a tem- 
porary storage structure, for starch or other substances 
may accumulate in the cells of this tissue during photo- 
synthesis. Specialized storage structures are extremely 
common among higher plants, and to such organs the 
food in diffusible form is transported. 



252 Plant Physiology 

It has been noted that certain mineral constituents 
migrate from old organs to the seed. In an analogous 
manner organic products are accumulated. Storage may 
also occur in bulbs, tubers, or aerial stems, roots, leaves, 
and fruits. It is, of course, such natural storage organs 
that have been seized upon particularly as food for man and 
feeding stuffs for animals, and many of the plants possess- 
ing these have been wonderfully improved through selec- 
tion and breeding. 

A storage organ of available food-material has a phylo- 
genetic reason for existence in the fact that it has to do 
with subsequent growth and fruiting or with the propa- 
gation of the species. Seeds, bulbs, tubers, and other such 
structures are essentially propagative devices, and it is 
not uncommon to find that they possess the capacity to 
lie dormant for a period, or to withstand desiccation. 

141. Annuals, biennials, and perennials. — These 
terms are used to imply one or more seasons of growth, 
When an annual plant, like the oat, reaches maturity by 
gradual, natural means, a very large part of the mobi- 
lizable carbohydrate and protein material is deposited or 
accumulated in the fruit or seed. The work of the plant 
as a whole is done, and there is no great wastefulness of 
readily used substance in the dead tissues. In the case 
of a biennial plant, such as the sugar-beet, which has grown 
for one season, the leaves have died, but the root has be- 
come an organ for a great accumulation of sugar and other 
food-materials. The next season by virtue of this stored 
food the energies of the plant may be vigorously directed 
toward the production and maturity of seed. 

The potato uses up during the early part of the season 



Metabolism; Digestion and Translocation 253 

not only the starch of the " seed tuber," but also practi- 
cally all of the starch which is made daily by the leaves, in 
the production of stem and leaf structures — additional 
starch-making surfaces. The growth of new tubers 
develops slowly at first, but finally the energy of leafy 
shoot " vegetation " wanes, and then a considerable sur- 
plus of carbohydrate is accumulated as starch in the 
tubers. In consequence, at maturity about 80 per cent 
of the dry matter of the tuber is starch. 

When a tree ceases to make food-material in the fall, 
there may be little or there may be much starch already 
accumulated. A peach tree, for example, heavily laden 
with young fruit in July may make each day a considerable 
quantity of starch. The latter may be found by the 
usual test applied to the leaves. The starch, however, is 
in considerable part used every day to furnish the carbo- 
hydrate used in the building of wood, in the making of 
fruit, and ultimately in respiration, so that only when the 
fruit is becoming ripe and the development of new wood 
is checked may there be a surplus of starch to accu- 
mulate in trunk and branches. After the ripening of the 
fruit much more starch may be made and accumulated 
in the twigs as a reserve for the young growth of another 
season. Such accumulations of food-material are indis- 
pensable, for in the peach thousands of blossoms are 
produced and the fruit set before the leaves are unfolded, 
a result of using food-materials that represent the work 
of the previous season. 

Many varieties of apple do not ripen until after the 
leaves fall, and it is possible that this holding and sustain- 
ing of the fruit so long (meanwhile using stored food- 



254 Plant Physiology 

material) may be a factor in the apparent tendency to- 
ward biennial fruit production, that is, to a greater 
production in alternate years. It stands to reason 
that a too heavy yield of fruit one season may have 
some effect upon the crop of the next year, although 
this tendency may be offset to a considerable extent by 
good culture, fertilization, and favorable season. 

142. Carbohydrates. — The carbohydrates include the 
sugars, starch, cellulose, and many of the other compounds 
containing carbon, hydrogen, and oxygen. They consti- 
tute the bulk of food-materials in general. In these sub- 
stances the molecule contains hydrogen and oxygen in 
the proportion of 2: 1, or as in water; and the number 
of carbon atoms is usually six or a multiple of six, but in 
some compounds five. The following are some important 
classes of these compounds: — 

(1) Monosaccharids (sugars), C 6 H 12 6 , including glu- 
cose, fructose, and galactose. 

(2) Disaccharids (sugars), CxoHooOn, including sucrose 
(cane sugar), lactose (milk sugar), and maltose. 

(3) Polysaccharids or amyloses, n (C 6 H 10 O5), including 
such compounds as starch, inulin, dextrin, glycogen, and 
cellulose. 

Briefly stated, the disaccharids and polysaccharids are 
for the most part readily convertible into monosaccharids 
through hydrolysis. This may be accomplished by boil- 
ing with acids, and also through the action of certain other 
compounds of metabolism, the enzymes, subsequently 
discussed. The transformation of cane-sugar may be 
represented as follows : — 

C^HooOn -\- H 2 = C 6 H 12 6 -\- CqH^Oq, 



Metabolism; Digestion and Translocation 255 

that is, producing dextrose and levulose. The hydrolysis 
of the polysaccharids may involve a series of changes in 
which water is taken up and ultimately n molecules of 
hexose sugar (or sugars) are split off. 

143. Sugars. — The commoner forms of sugar found 
in plants are sucrose (cane-sugar), glucose (dextrose), 
and fructose (levulose). Glucose and fructose are ap- 
parently important products in the metabolism of cells 
usually, but these compounds are often promptly used 
in general metabolism, especially in the building up of 
other products. There may be no accumulation of them 
in the plants where they are being constantly manufac- 
tured. Accumulation does occur, however, particularly 
in ripening fruits, such as the grape, peach, prune, and 
date. In the raisin the characteristic brown nodules of 
this sugar may be seen. Indirectly these sugars are of 
much commercial value, for sweetness and flavor together 
determine the prices paid for fruits, prices which are in 
general far above their actual food value. 

The monosaccharids are reducing sugars, precipitating 
heavy metals from solutions of their salts upon heating. 
Maltose also possesses this quality, but cane-sugar does 
not. A standard test for reducing sugars is obtained by 
Fehling's solution (see Laboratory work), consisting of 
copper sulfate in an alkaline solution of potassium sodium 
tartrate. Upon boiling with this solution a brick red 
precipitate of cuprous oxide is produced. 

Sucrose is the form in which sugar commonly accumu- 
lates in plant cells. From the stems of the sugar cane and 
from the root of the sugar-beet there were extracted during 
1909 nearly 15,000,000 tons of commercial sugar. The 



256 Plant Physiology 

juices of selected races and strains of the two plants indi- 
cated may contain from 14 to 18 per cent of sugar, and the 
history of the breeding of the highly productive races of 
the sugar-beet is of special physiological interest. Sor- 
ghum and. a few species of tropical palms also contain 
cane-sugar in sufficient quantity to be of commercial im- 
portance. Sugar maple {Acer saccharum) growing in 
northern latitudes yields a sap which in the usual time of 
cupping (late winter or early spring) may contain from 
2 to 5 per cent of cane-sugar, besides other substances 
imparting the peculiar flavor for which this sugar is prized. 

144. Starches. — The great majority of green plants 
produce starch (Fig. 65). There are some exceptions 
among several orders of flowering plants, especially certain 
monocotyledons (Sect. 112) ; and certain groups of alga? 
do not possess this capacity, notably the families of blue- 
greens and browns. 

The starch molecule is very complex and difficult of ex- 
act study, but the occurrence and reaction of the starches 
are well known. Starch occurs in the form of insoluble 
grains with a characteristic general appearance, varying 
considerably, however, in form, size, and markings in the 
different plants in which produced. The grains may be 
simple, semicompound, or compound. Very large grains, 
often so large as to be visible to the unaided eye, are found 
in the root-stock of Canna ; those of potato are of medium 
size; in rice the components of the compound grains are 
small and numerous ; while in spinach they may be ex- 
tremely minute, and according to Nageli as many as 30,000 
may be united together. 

Starch grains are produced within plastids, — chloro- 



Metabolism; Digestion and Translocation 257 



plasts and amyloplasts (leucoplasts), — and the exact 
method of formation is imperfectly understood. It is a 
general belief that starch 
is formed from glucose 1 
and under the influence of 
one or more enzymes. 
The latter are believed to 
be active in starch pro- 
duction, as a rule, when 
carbohydrates exist in the 
cell in considerable excess 
of use. 

Schimper considers the 
starch grain to be a 
spherite made up of a 
multitude of needle-like 
crystals radiating from 
the center. The striated appearance commonly ex- 
hibited seems to be due to difference in nutrition during 
the formation, and the eccentric arrangement of the mor- 
phological center may be due to the development of the 
grain near the periphery of the plastid. 

Starchy products constitute a very large portion of the 
food of man and of domestic animals, so that many prod- 
ucts are valuable chiefly from the high starch content. 
In passing from colder to warmer regions some of the more 
important starch-producing plants are the following : 
the small cereals, buckwheat, corn, beans, potatoes, sweet 
potatoes and yams, cassava, rice, yautia, arrow-root, 

1 The view is also current that the starch molecule is split off from 
protein material of the plastid. 




Starch grains 
forms. 



258 Plant Physiology 

sago, tapioca, bread-fruit, banana, and many other vege- 
tables and fruits. Reckoned in per cent of dry weight, 
the potato tuber contains a starch content of about 80 
per cent, while corn and many cereals may contain 60 
per cent or more. According to Konig starch contains on 
the average about 15 per cent water, 1 per cent nitrogenous 
bodies, and generally much less than 1 per cent of ash. 

Upon hydrolysis starch yields, as subsequently shown, 
first dextrins, then maltose, and this is ultimately trans- 
formed to glucose, although some of the dextrin (about 
one fifth) is more resistant to hydrolysis. 

Inulin, an amylase less complex than starch, is char- 
acteristic of the tuberous roots of Dahlia and of some other 
composites, although occurring also in other plants. It 
is dissolved in the cell-sap, but may be crystallized out as 
spherites. These crystals are soluble in hot water. It 
yields fructose on hydrolysis. 

145. Cellulose. — The cell-walls consist in large part 
of a substance which passes under the general name of 
cellulose w(C 6 H, O 5 ). Cell-walls are frequently impreg- 
nated with gummy, metallic, or other substances ; this 
is the usual case with epidermis, cork, wood, and the like. 
Nevertheless, some form of cellulose forms a large per cent 
of the walls of flowering plants. 

The celluloses proper resist hydrolysis with weak acids, 
and except at the time the cell-walls are being laid down 
they are unimportant in metabolism. Hemicelluloses are 
forms which are readily hydrolyzed, yielding monosac- 
charids other than glucose. They constitute, in fact, 
the reserve cellulose deposited upon the cell-walls in the 
endosperm of many seed and some other storage organs, 



Metabolism; Digestion and Translocation 259 

especially in the seeds of palms. This reserve food be- 
comes available during germination. 

146. Fats and oils. — Fats and oils are far more com- 
mon and important constituents of plants than is popu- 
larly supposed. In Liliaceae and a few other mono- 
cotyledonous orders oils replace starch as the first visible 
photosynthetic product. Oily bodies occur in active cells 
often as small droplets in the cytoplasm. In a variety of 
seeds the amount of fatty substances is considerable, a 
part occurring as globules or as crystals. 

Among the more important fats and oils may be men- 
tioned those from corn, coconut, various palms, olives, 
mustard, poppies, flax, castor-bean, Bergamot-orange, 
carnation, Brazil-nut, cotton, etc. Thousands of tons of 
palm and coconut oil are annually imported into Europe 
and constitute an important item of trade. The value 
of the cotton-seed oil produced during 1909 and 1910 is 
estimated to have been upwards of $300,000,000. 

Oils and fats may be identified by comparatively simple 
tests and they are obtained for commercial purposes by 
crushing and pressing, or by extraction. 

147. Proteins. — The vegetable proteins are numerous, 
and they vary greatly in physical and chemical character- 
istics. They may occur in solution in the cell vacuoles, 
partially dissolved in intimate association with the pro- 
toplasm, and as solid forms — crystals or granules. The 
latter occur especially in storage organs or tissues with 
reduced water-content, usually associated with carbohy- 
drate storage products, oil, and other substances, as in 
many legumes. 

The vegetable proteins have been studied more particu- 



260 



Plant Physiology 




Fig. 66. Endosperm cell (.4) of Ricinus in 
water; aleurone grains (B) in olive oil 
protein crystal (k) and globoids. (g). [Afte; 
Strasburger.] 



larly in the seed, so that the storage forms are better 
known. The aleurone grains of the endosperm of cereals 

are familiar protein 
bodies, and in this 
case they are found 
abundantly in the 
outer layer of the 
endosperm, while 
starch is more abun- 
dant within. 

The gluten of 
wheat consists of a 
variable mixture of 
proteins. Flour may 
contain about 10 per cent of this material and about 
70 per cent of starch. The gluten is readily separated 
from the starch in a proximate manner by kneading the 
flour under water in a thin cotton bag. In this manner 
the flour is filtered out and the gummy nitrogenous sub- 
stance remains in the bag. Hard wheats are particularly 
valuable in the manufacture of products like macaroni 
(also in bread-making) as a result of the relatively high 
content and composition of the gluten. 

148. Classes of proteins. — A system of classification 
of the proteins in keeping with that of other chemical sub- 
stances (the molecular structure of which is better known) 
is now impracticable. Classification is based largely upon 
solubility under certain standard conditions. Three 
principal groups of proteins are recognized, namely, (1) 
simple, (2) conjugate, and (3) derived. 

The simple vegetable proteids include albumins, some 



Metabolism; Digestion and Translocation 261 

of which occur in seeds and in cell-sap, such as .leucosin of 
many cereals, legumelin of legumes, and ricin of the castor- 
bean. Such proteins are generally soluble in water. 
Globulins are soluble in salt solutions, but practically in- 
soluble in pure water ; such are legumin of many legumes, 
amandin of nuts, tuberin of the potato, and many others. 

The conjugate proteins are so named because of the 
apparent association of two substances in the molecule 
(nucleic acid and protein in the case of nucleo-proteins) 
or of the ready separation of the molecule into these two 
substances. The nucleo-proteins are of much importance 
as constituents of nuclei. 

Derived proteins, such as the proteoses and peptones, 
are considered more particularly under digestion, and 
may be regarded as the digested and diffusible products 
demanded by the cell for direct use in assimilation. 

149. Amides. — Amides are also well known in plant tis- 
sues. Among these asparagin is of frequent occurrence. 
As a rule these compounds are not a storage form of nitro- 
gen. They are commonly produced, and may accumulate 
to a considerable extent, during germination ; from 10 to 
30 per cent of the nitrogen in this form is not infrequent 
at that time. Leguminosse may contain these com- 
pounds in exceptional amount, 75 per cent of the total 
nitrogen in vetch during germination being thus reported 
by Schultze. It may be regarded as a degradation prod- 
uct of protein, a product which is readily diffused, and 
again used. 

150. Organic acids. — Organic acids are common con- 
stituents of plant juices. They may occur free or com- 
bined with mineral bases. As a result of the presence of 



262 Plant Physiology 

free acids and acid salts the cell-sap may be acid in 
reaction. 

These substances may be looked upon usually as the 
by-products of metabolism, but they may be serviceable 
and some may function further in general metabolism. 
The group of fatty acids is well represented among the 
compounds in plant tissues ; but the acids of commoner 
occurrence are the related oxalic, malic, tartaric, and citric, 
all being oxidation products of glycols (dihydroxy-deriva- 
tives of the paraffins). 

Oxalic acid is of widespread occurrence, and it is most 
familiar (in the form of calcium oxalate) as the raphides 
or needle-shaped crystals so common in many vegetative 
organs. Malic acid is well known in many unripe poma- 
ceous and stone fruits, but it occurs far more commonly, 
especially in " fatty " plants like the stone-crops ; and it 
is the substance found by Pfeffer and others to be chiefty 
responsible for the " attraction " directing motile sperms 
to the egg-cells of certain ferns. Tartaric acid is readily 
extracted from the grape, in which fruit it occurs as acid 
potassium tartrate. Citric acid may constitute from 
6 to 7 per cent of the juice of lemon, and it is also more 
abundant in the other species of Citrus than in higher 
plants generally. 

The production of acids is usually favored by the abun- 
dance of soluble carbohydrates in the tissues. Submersed 
in a solution of glucose, for instance, the leaves of Oxalis 
rapidly increase in acidity. Among the lower plants, 
fungi and bacteria, the production of organic acid is even 
more common than with the higher plants, as again referred 
to under fermentation phenomena. 



Metabolism ; Digestion and Translocation 263 

151. Tannins. — The tannins are bitter, astringent, 
water-soluble, amorphous substances widely distributed in 
the leaves, bark, and fruits of plants. All of these sub- 
stances, which are of commercial importance, may be em- 
ployed in the process of tanning skins and hides, since they 
form insoluble compounds with various nitrogenous bodies, 
giving a toughness and durability to the skin which con- 
stitutes the differences between leather and natural skin. 

The tannins are alike in certain physical and chemical 
properties, but there may be dissimilarity in chemical 
composition. The tannin (glucoside) used in the making 
of leather is usually derived from the bark of various trees, 
including that of hemlock and oak, so extensively employed 
in the United States. The bark of hemlock may yield 
from 8 to 10 per cent of its dry weight of tannin and the 
leaves of tea may contain as much as 15 per cent. 

Tannin is also extensively used as a mordant in the 
process of dyeing, for it produces colored products with 
various dye-stuffs ; and it has long been employed in the 
manufacture of ink. The characteristic purplish brown 
color of the trunk of the cork oak from which the bark 
has been removed is due to this substance. The chief 
source of tannic acid (digallic acid) is a gall-nut produced 
upon an oak (Qaercus infectoria) , a product obtained for 
the most part from Turkey. Tannic acid constitutes 
more than one half of the dry weight of this gall. A 
similar product is yielded by a gall upon the sumac, Rhus 
semi-alata, which occurs in China. 

Upon heating with sulfuric acid, tannic acid is 
hydrolyzed, yielding two molecules of gallic acid, thus, 
C 14 H 10 O 9 + H 2 = 2 C 7 H 6 5 . This process is also ac- 



264 



Plant Physiology 



complished naturally by means of a few fungi, especially 
Aspergillus niger. 

152. Resins and turpentine. — A great variety of 
products of physiological interest and of commercial im- 
portance are included in the groups commonly called 
resins and turpentine. They are produced in the cortex 
and young wood of a variety of plants generally charac- 
terized by special ducts or canals formed in connection 
with the conduction of these products. 

The conifers furnish the chief commercial supply, and 
they constitute an important economic item in many of 
the coniferous forests of Europe and America. For a long 
time the balsams, especially the Canada balsam, have been 
a product of northern forests, whereas the turpentine in- 
dustry has been best developed in the Southern States. 
According to Mayr a cubic meter of the splint wood of the 
standing tree contains approximately the amounts of 
fresh resins named, of which turpentine oil constitutes a 
considerable percentage, as follows : — 





Resins 


Turpentine Oil 


Pinus silvestris 

Larix europsea 

Picea excelsa 

Abies alba 


21.1 

18.3 

9.4 

3.2 


60.0 
33.1 
38.2 

32.4 



It is thus evident that the hemlock, which is poorest in 
solid resins, contains a very large per cent of the product 
as turpentine oil. The resins belong to the terpene series, 
but they occur along with various acids and other com- 






Metabolism ; Digestion and Translocation 265 



pounds. Turpentine 
oil contains a large 
per cent of various 
volatile oils. The 
common method of 
turpentine orchard- 
ing, has resulted in a 
great loss of timber 
due to the severe in- 
jury to the tree from 
the boxing and 
chipping employed. 
Methods have been 
suggested whereby 
this injury is now re- 
duced to a minimum 
(Fig. 67), The 
crude products ob- 
tained by the 
method indicated are 
distilled, the volatile 
spirits being con- 
. densed, constituting 
turpentine, while the 
nonvolatile products 
are the solid rosins 
of commerce. 

153. Digestion. — 
The seed and the 
tuber are effective 
propagative devices, 




Fig. 67. Turpentine orcharding. 
[After Forest Service.] 



266 Plant Physiology 

because of the fact that they are at the same time storage 
structures. When, subsequently, conditions become 
favorable for the growth of the seedling or of the sprout, 
the seed or tuber is exhausted of its stored substances, 
which again move to the growing organs. 

The starch and many other reserve foods stored in the 
tuber or in the endosperm of the seed are insoluble and 
indiffusible. It has already been indicated that for storage 
purposes solid or indiffusible forms may be necessary or 
economical. Nevertheless, such reserve foods, or the sub- 
stances from which they were formed, enter the storage 
cells as diffusible products, and in such forms only can they 
find exit. The process of rendering organic materials 
soluble and diffusible, that they ma}' be used in the cell 
or transferred to other cells and organs, or used in the 
building up of new substances, is digestion. It is a cata- 
bolic or breaking-down process, and the special nature of 
the changes involved is as diverse as the products acted 
upon. 

From the preceding it is evident that when starch or 
any other insoluble food product is formed in the cells 
which are actively engaged in photosynthetic work, these 
products must undergo digestion before use or removal. 

154. Digestion in different organisms. — Digestion in 
any cell or organ, in the animal or in the plant, is the same 
in principle. It is generally accomplished or accelerated 
by means of certain nitrogenous bodies or enzymes (in- 
cluded among catalytic agents) secreted by the protoplasm 
of the storage cells or the cells in the vicinity. In the ver- 
tebrate animal, digestion is effected through the secretion 
of digestive enzymes which enter the alimentary tract, 



Metabolism; Digestion and Translocation 267 

for the food-stuffs must be made soluble prior to direct 
absorption by the cells of the body. The parasitic fungus 
or bacterium may be able to dissolve and to penetrate the 
cell-wall. Then upon entering the cell the fungus may also 
gradually " appropriate " or digest the starch and other 
foods, absorbing them in this case, as does the higher ani- 
mal, after digestion. It matters not what the organism 
may be which digests starch, the method is the same, 
and it is dependent upon the ability of the organism under 
the conditions to produce and often to secrete the starch- 
digesting or the starch-splitting enzymes. The same 
applies to other solid or indiffusible food substances, so that 
in general it may be said the use of all such substances 
as food is a factor of the specific digestive capacity of the 
organism, however simple or elaborate the digestive 
apparatus may be. 

After all, the whole phenomenon of nutrition of even the 
green plant is not essentially different from that of the 
animal. The green plant makes its carbohydrate foods 
in certain cells, and it builds up nitrogenous substances out 
of these and inorganic nitrogen ; but once sugar and nitrog- 
enous bodies are formed, nutrition follows a course com- 
parable in the two. 

155. Enzymes and enzyme action. — The enzymes 
or soluble ferments are doubtless exceedingly numerous, 
and possibly the digestive enzymes alone are almost as 
many as the different kinds of reserve foods. So far as 
the study of these substances has progressed, they seem 
to be, for the most part, of protein character. At any 
rate, they are precipitated with proteins, yet certain 
analyses of the purified products disclose no nitrogen con- 



268 Plant Physiology 

tent, and much doubt is entertained respecting their pre- 
cise nature. In general, however, they are regarded as 
protein. They show noteworthy differences among 
themselves, with regard to solubility, conditions of precipi- 
tation, and the like. 

The enzymes are products of protoplasmic activity 
and are not generally regarded as readily diffusible ; that 
is to say, the work of many of these is primarily within the 
cell where they may be produced. These are the intra- 
cellular enzymes. Nevertheless, in a number of cases 
there may be specialized secretory cells of some impor- 
tance ; and in other cases the digestion, or partial diges- 
tion, of products prior to absorption, is an indispensable 
character, as in the case of fungi and higher animals gen- 
erally. Those enzymes which are active in part without, 
or beyond, the limits of cells producing them are termed 
extracellular enzymes. 

The hydrolysis or decomposition of organic bodies like 
protein, starch, and fat under laboratory conditions (other 
than by the use of enzymes) is effected only by means of 
fairly strong acids, high temperatures, and other intensive 
agents. Contrariwise, the enzymes effect hydration and 
decomposition under the conditions of the plant cell or 
body, although their activity frequently reaches a maxi- 
mum at 40° C. or slightly above. 

A great majority of the commoner enzymes act by 
hydration; thus the effect of invertase upon cane-sugar 
is as follows : — 

C12H22O11 -j- H2O = CeH^Oe -\- CeH^Oe- 

Sucrose Dextrose Levulose 

On the other hand, certain classes of enzymes, not here 



Metabolism; Digestion and Translocation 269 

particularly considered, are believed to act by oxidation, 
and simple, molecular decomposition may also occur. 
The products may be diverse, as is common, or alike, as 
when maltose is transformed into two molecules of 
dextrose. 

From relatively recent work it has also become certain 
that enzymes are important in synthetical processes as 
well as in the analytical ways referred to. In the former 
case they are said to possess a reversible action ; that is, 
for example, — contrary to the instance above cited, where 
cane-sugar may be hydrated with the production of hex- 
oses, — the hexose molecules may be built up by means of 
an enzyme into the anhydride or disaccharid form. Re- 
versible actions appear to be very common, but little 
definite information is available at the present time. 

Direct sunlight is promptly injurious to enzyme action. 
Fermentation is also commonly weakened at temperatures 
above 50° C. and " death " may result above 70° C. 
Most toxic agents are injurious to enzyme action at con- 
centrations much above the normal death-point of the pro- 
toplasm, but at considerable dilution acids or alkalies may 
be stimulating. While a weak percentage of alcohol and 
a saturated solution of chloroform may not be injurious, 
strong alcohol may be fatal, so that in the precipitation of 
enzymes by 95 per cent alcohol there may be danger of 
losing the product. 

156. Carbohydrate enzymes and their products. — 
Of the many carbohydrate enzymes it is possible here 
briefly to consider only a few. Chief in importance among 
these are diastases (amylases), acting upon starch, the 
hydrolysis and splitting of which yields a series of dextrins, 



270 



Plant Physiology 








,<^ 



.<?:,- 



P:J 



Fig. 68. Corrosion of starch grains by dias- 
tase of secretion. [After Strasburger.] 



and finally maltose — which is subsequently converted 
by the enzyme glucase into glucose. 

The diastases are widespread, and two forms are dis- 
tinguished accord- 
ing to the type of 
corrosion of the 
starch grain. 
Diastase of trans- 
location occurs es- 
pecially in the 
chlorenchyma of 
leaves, and it cor- 
rodes the grain 
almost evenly. 
Diastase of secre- 
tion, corroding the 
grain irregularly (Fig. 68), is that which occurs in storage 
organs generally, but especially in seeds. Apparently a 
third form, takadiastase, is the product of Aspergillus 
Oryzce in its action upon wheat or rice starch. Allied to 
diastase is inulase, converting inulin into fructose. 

Cytase is a ferment often associated with diastase, as 
in the endosperm. It may be important in the dissolution 
of a certain " shell " of the starch grain. It is, however, 
best known from its action on the reserve cellulose, con- 
verting this into hexose sugars. It is probable that 
several enzymes are required in the decomposition of other 
celluloses, about which very little is known. 

It has been indicated incidentally that the disaccharids 
sucrose and maltose are hydrolyzed and yield hexoses by 
invertase and glucase respectively. These enzymes are 



Metabolism ; Digestion and Translocation 271 

widely distributed in both higher and lower plants. Both 
these and the diastases are most important in the produc- 
tion of malt used in brewing and distilling. The diastases 
are also important medicinally as an aid to amylose diges- 
tion, and many patented forms are on the market. 

Another enzyme of special interest requiring further 
study is pectase, a form important in the hydrolysis of a 
portion of the cell-wall, producing a jelly from the pectic 
compounds. 

157. Protein enzymes. — Protein enzymes were among 
the first to receive attention, and they have been more 
completely studied in the animal organism, where the 
action of pepsin in the stomach and that of trypsin 
received into the intestine from the pancreas are well under- 
stood. These enzymes must occur in plants, or else 
others which serve the same purpose. 

Through ferments many proteins are converted into the 
more diffusible proteoses and peptones; while tryptic 
ferments may give a more complete digestion, reducing 
the peptone to the readily diffusible amido and amino 
acids, such as leucin and asparagin. Ferments which 
have been regarded as tryptic have been known for some 
time in plants, such as papain from the papaw, and bro- 
melin from the pineapple. From recent work it appears 
that some of the so-called tryptic ferments may be, in fact, 
combinations of peptic and ereptic ferments. The last- 
named class is found by Vines to be well distributed in 
plants, and it decomposes the peptones with the pro- 
duction of amido and amino acids. 

Doubtless many of the protein enzymes in plants are 
intracellular. The carnivorous plants, such as the sun- 



272 Plant Physiology 

dew and Nepenthes, secrete enzymes which act from with- 
out the absorbing organs, and likewise the fungi produce 
enzymes at least slowly diffusible. The exact method 
of action of the protein enzymes is not yet clear, but it is 
generally assumed to be hydrolytic. It is necessary that 
protein enzymes be produced in plants in some quantity 
at the time of germination to effect the movement and use 
of stored products, for, as has been shown already, indif- 
fusible protein compounds are common in such organs. 
Moreover, at the maturity of the plant or of any vegeta- 
tive organs of the plant much of the solid protein and pro- 
toplasmic material is converted and accumulated in the 
seed, and in the recovery of this from organs which have 
ceased to grow there is, of course, much economy. 

158. Conduction of digested foods. -From what has 
been said regarding the action of digestive* enzymes, it is 
apparent that there is required an effective means of trans- 
location, as may be demanded, for digested materials to 
and from the leaves, shoots, storage organs, and seeds. 
Diffusible organic substances in complex plants seem to 
require, then, for rapid diffusion to and from active organs 
specialized paths or tissues. This demand is met through 
the phloem of the vascular bundles, in which the sieve 
tubes occur (Fig. 6,n,o). The large protoplasmic connec- 
tions between the rows of sieve cells evidently permit a 
movement more rapid than simple diffusion. 

The sieve tubes are important in the movement of such 
products as shown by direct and indirect evidence : thus, 
by the fact that sieve tubes in particular contain a quan- 
tity of simple organic substances ; by the absence of such 
products in so great a quantity in xylem, pith, or cortex; 



Metabolism; Digestion and Translocation 273 

by the interruption of conduction upon the removal of 
the phloem; and by the failure of the loss of cortex to 
affect directly this movement of organic substances. 

159. Ringing. — In horticultural practice ringing is ap- 
plied to the removal of a small band of bark encircling the 
stem of dicotyledonous plants so as to include the cortex 
and phloem. Sometimes there is made merely a circum- 
ferential cut through to the wood ring, or a small wire is 
bound tightly about a limb so as to cut into the bark, 
but these latter may heal too quickly to effect the result 
desired. 

The principle is evident. Ringing interrupts the more 
rapid movement of digested foods toward the roots or 
basal parts to the detriment and often to the ultimate 
death of these structures ; but there is an accumulation of 
foods above the ring, and this may be favorable for the 
developing fruit. This operation may result in a consider- 
able increase in the size of the shoot, or in the production 
of tumors, above the incision. 

Ringing is reported a widespread practice in Europe 
with grapes and apples, and it is employed to some extent 
in the United States. It should be used with caution where 
the plant is expected to serve future usefulness. It ma3 r , 
however, increase or incite productivity, hasten ripening, 
or enhance the size and quality of fruit. In the latitude 
of New York the grape is generally ringed during late 
June. The place of ringing should be between the chief 
fruiting canes and the main vine, but the exact location 
will be determined by the system of training. There 
should be such a development of canes below the ring 
as to fairly well nourish the main vine and root system. 



274 Plant Physiology 



LABORATORY WORK 

Starch. — Examine under the microscope and describe the 
starch grains from a variety of sources, such as Canna (root- 
stock), potato (tuber), rice or oat kernel, milky juice of Euphor- 
bia, and seed of beet. Leucoplasts associated with starch grains 
may be best observed in fixed and stained material, but they 
are also visible without staining in such favorable material as 
the young shoots of Canna, or in the young root-stocks of various 
monocotyledons. 

Rub up about 1 gram of starch with a small quantity of water 
in an evaporating dish, and when there are no more lumps dilute 
to 50 cc. Is starch soluble in cold water? Heat the preceding 
to the boiling point, and when a paste is formed, examine it 
microscopically with respect to solubility. 

With the paste above prepared, and with a weak alcoholic 
solution of iodine, make a complete test of the iodine reaction. 
In small test-tubes first use a few drojx of a si rone: paste and 
considerable iodine solution, then weaken the paste up toward 
a dilution of one hundred times, using also less or weaker iodine. 
Determine the effect of heating and recooling, also of a few drops 
of strong caustic potash, upon the iodine reaction. Compare 
the reactions of the starch paste toward iodine with that of a 
suspension of starch in cold water. 

Study the distribution of Btarch in any plant available, em- 
ploying sections, especially from fleshy roots. Leaves, etc Deter- 
mine where in the resting twigs of apple, lilac, or maple the 
storage of starch occurs. In order to stain starch occurring in 
small quantities in the tissues, especially in the cells of 1< a 
as of Elodea grown in weak light, or to bring out the starch in 
chlorophyll bodies, the material may he stained in a concentrated 
solution of iodine in potassium iodide, when the grains stand out 
black. Again, a dilute solution of iodine in potassium iodide 
may be used, and after washing, the material may he laid in a 
strong solution of chloral hydrate which dissolves most of the 
cell-contents, swells the stained grains, and in time decomposes 
these last also, so that a prompt examination must be given. 



Metabolism; Digestion and Translocation 275 

Inulin. — Make and examine sections (mounted in alcohol) 
from small pieces of the tuberous roots of Dahlia which have 
lain for a week or 10 days in strong alcohol. Describe the 
spherites observed. Treat the sections with cold water and ex- 
amine, then treat with hot water, and discuss solubility. 

Glucose and other sugars. — The most decisive test for glucose, 
other reducing sugars, and certain glueosides is the precipitation 
of cuprous oxide in Fehling's solution. The identification of the 
different sugars or other substances may require other tests. 

Prepare Fehling's solution using two bottles as follows: A, 
34.6 grams of pure crystals of copper sulfate dissolved in distilled 
water and made up to 500 cc. ; B, 173 grams of Rochelle salt 
(potassium sodium tartrate) and 60 grams of sodium hydrate 
dissolved in distilled water to make 500 cc. In employing this 
test use always equal quantities of the two solutions. 

Add to some Fehling's solution in a test-tube a small granule 
of glucose or a few drops of a strong solution. Boil the solution 
for three minutes, and describe the reaction. In the same manner 
test the juice of ripe grapes, or ripe plums or peaches. In this 
case note the rapidity of the reaction, to contrast with a later 
test of beet juice. 

Use the Fehling's solution with a few crystals of cane-sugar. 
Is there any reduction? Boil the cane-sugar previously with a 
few drops of hydrochloric acid, neutralize with KOH (to litmus), 
and then repeat the Fehling's test. Discuss. Press out some 
juice of the sugar-beet, or grate up a small amount, and extract 
with water; then test this juice (or extract) with Fehling's 
solution, being careful to heat gently, since violent heating will, 
through other substances present, be alone sufficient to convert 
cane-sugar. Compare the result with that obtained when grape 
or peach is employed. Compare the reaction of crystals of cane- 
sugar and granules of glucose in a few drops of concentrated 
sulfuric acid. 

Celluloses. — Crack a seed of date, make a section or shaving 
of the endosperm, and study the preparation with respect to 
the reserve cellulose deposited upon the cell-walls, describing 
accurately the nature of the cells in which such deposits occur. 



276 Plant Physiology 

Test the solubility of cellulose (cotton fibers) in concentrated 
sulfuric acid and in cuprammonia. In the first case use small 
quantities of the materials, rub up in a Syracuse watch glass, 
when dissolved neutralize with KOH and test for reducing sugar 
(glucose). 

With half-concentrated sulfuric acid determine the length of 
treatment required to yield a blue color with iodine. 

Place cotton fibers, sections of a root-tip, etc., in a solution of 
chloriodide of zinc (dissolve chloriodide of zinc in less than its 
weight of water and add metallic iodine until a bright cherry 
color is produced). Place the material in the concentrated solu- 
tion, examine under the microscope, and describe the character- 
istic color reaction. 

Fats and oils. — The fats and oils are generally soluble in 
ether, chloroform, benzene, and other solvents of this nature, 
and certain oils (castor oil) in absolute alcohol. Examine 
sections of the endosperm of castor-bean and of the garden bean 
in water ; then after immersion for a few minutes in absolute 
alcohol and ether, reexamine. Stain similar sections from a few 
minutes to half an hour with a 50 per cent alcoholic solution of 
cyanin (or in a solution of alcanna in absolute alcohol, then 
diluted to 50 per cent) and note the deep color of the oily bodies. 

Proteins. — Proteins may be soluble in water, in salt solutions, 
in alcohol, and in acids and alkalies. Make sections, or shave 
off with the razor fragments from the endosperm of wheat, 
mount in water and examine for "aleurone " grains (not soluble 
in water) in the outer layer particularly. In the same way 
examine sections of the endosperm of castor-bean for protein 
crystalloids and globoids, preferably after removing the oil (in 
this case) by immersion for a few minutes in absolute alcohol. 

Mix some wheat flour and water, place in a cloth bag and 
knead under a stream of water at the faucet. The glutinous 
dough resulting after the starch is washed out is the gluten of 
wheat, consisting of a mixture of protein substances some of 
which, in the living cells, are indistinguishable from the cyto- 
plasm. Take a small portion of this gluten, rub it up with a 
2 per cent salt solution, and save for later study. Test the sol- 



Metabolism ; Digestion and Translocation 277 

ubility of another portion of the gluten in 70 per cent alcohol 
and save the solution. Use a part of this solution for an obser- 
vation upon coagulation by diluting the part taken to three times 
the volume with distilled water. 

Grind up three grams of beans extracted with 30 cc. of water 
in a test-tube, shake occasionally for 10 minutes, filter, and 
employ the filtrate along with the preceding solutions in some 
of the tests given below. A portion of this filtrate, however, 
may be tested as to coagulation in two ways : (a) acidulate a 
small amount in the test-tube and apply heat ; (6) add to a 
few cubic centimeters in a test-tube four times the volume of 
95 per cent alcohol. 

The following are some reactions of proteins which are in 
part distinctive : — 

1. Brick or rose-red color with Millon's reagent on standing, 
or with gentle heat. [To prepare Millon's reagent dissolve 1 
gram of mercury in 2 grams of nitric acid (1.42 s. g.), and then 
dilute with twice the volume of water.] 

2. The Biuret reaction, a violet or purple color with copper 
sulfate and sodium hydrate. To a very weak copper sulfate 
solution add excess of potassium hydrate and apply heat, then 
add a small amount of a protein solution and heat again. 

3. Yellow color on boiling with nitric acid, the xanthoproteic 
reaction. After boiling, cool under the faucet and add ammonia, 
when the color will change to orange. 

4. Violet color with acetic and sulfuric acid. Use 2 parts 
glacial acetic and 1 part sulfuric acid, with a small amount of 
protein material, and apply gentle heat. 

Starch digestion. — Several forms of diastase may be obtained 
as commercial products, but it is well to undertake the extraction 
of one or more of these in the laboratory. Take 200 grams of 
clean barley seed, soak in running water over night, and germi- 
nate in a thick layer over moss until the roots are about 1 inch 
long and the plumules well started. Dry at 40 or 50° C. for 
about six hours. From this malt the diastase of secretion is to be 
obtained. In the same way collect (preferably an hour or two 
at least after sunset) about 200 grams of nasturtium, bean, or 



278 Plant Physiology 

potato leaves and dry in the same manner. These will yield 
diastase of translocation (ungerminated seeds of barley likewise). 
Grind the products separately (also powder the leaves) and ex- 
tract for 24 hours with two times the weight of 20 per cent alco- 
hol. Filter the alcoholic extract and precipitate the crude 
diastase (along with some other proteins) by adding 2\ parts 
or more of 95 per cent alcohol. After a short time filter to collect 
the precipitate. Dissolve the first (from barley seed) in 50 cc. 
of water, and the second (from leaves) in 25 cc, adding to the 
first 2 cc. chloroform, and to the latter 1 cc, to inhibit the 
growth of bacteria. 

Make about 200 cc. of 1 per cent starch paste (from rice 
starch), and into each of three test-tubes pour 25 cc of the 
starch paste, labeling the tubes, A, B, and C. 

To tube A add 5 cc. secretion diastase. 

To tube B add 5 cc translocation diastase. 

To tube C add 5 cc. distilled water. 

At intervals of \ hour shake the tube, take 1 cc. samples, and 
test with iodine, noting the changes of color (as dextrins are 
produced) from blue through purple to wine-red, finally colorless. 
When the starch reaction has disappeared, note the change in 
appearance of the solution. Test with Fehling's solution for 
reducing sugar. 

Drop a small quantity of potato starch into a few cubic centi- 
meters of each of the two diastase solutions (translocation and 
secretion) in two vials, slightly acidulate with weak HOI, and 
after intervals of an hour or so study the types of corrosion. 

If time permits, compare the effects of high and low tempera- 
ture, bright and reduced light, and strong and weak acids upon 
diastatic action. 

Translocation. — Verify the previous indications respecting 
the use of starch by the leaf or loss from the leaf when placed in 
the dark. Employing a Fuchsia (as previously used, page 223 1 , 
geranium, or nasturtium, secure a plant which has been exposed 
to bright light three hours or more, so that abundant starch occurs 
in the leaves. Select tw^o or three healthy leaves and on one 
side of the midrib, or middle, of each sever completely the main 



Metabolism; Digestion and Translocation 279 

nerves or veins. Place the plant in the dark for four hours, 
or over night, then dissolve out the chlorophyll, apply the starch 
test, and discuss the results with respect to translocation. 

References 

Armstrong, E. F. The Simple Carbohydrates and the Gluco- 

sides. 112 pp., 1910. 
Bayliss, W. M. The Nature of Enzyme Action. 90 pp., 1908. 
Cross, C. F., and Be van, E. J. Researches on Cellulose, 1895- 

1900. 180 pp., 1901. 
Czapek, F. Biochemie der Pflanzen. 1 : 584 pp., 1905 ; 2 : 1026 

pp., 1905. 
Dox, A. W. The Intracellular Enzymes of Penicillium and 

Aspergillus. Bur. An. Ind. Bui. 120 : 70 pp., 1910. 
Effront, J. Enzymes and Their Application. Vol. 1. The 

Enzymes of the Carbohydrates. (Transl. by S. C. Pres- 

cott.) 322 pp., 1902. 
Fischer, E. Untersuchungen liber Kohlenhydrate und Fer- 

mente. 1884-1908. 
Green, J. R. The Soluble Ferments and Fermentation. (2d 

Ed.) 512 pp., 1901. 
Hedrick, U. P., and Wellington, R. Ringing Herbaceous 

Plants. N. Y. Agl. Exp. Sta. Bui. 288 : 17 pp., 4 pis. 
Meyer, A. Untersuchungen iiber die Starkekorner. 318 pp., 

99 figs., 9 pis., 1895. 
Osborne, T. B. The Vegetable Proteins. 125 pp., 1909. 
Paddock, W. Experiments in Ringing Grape Vines. N. Y. 

Agl. Exp Sta. Bui. 151 : 8 pp., 3 pis., 1898. 
Vines, S. H. The Proteoses of Plants. Ann. Bot, 18 : 289- 

318; 19:149-162; 19:171-188; 20:113-122; 22:103- 

114; 23: 1-18. 
Wiesner, J. Die Rohstoffe des Pflanzenreichs. 1 : 795 pp., 

453 figs., 1900; 2:1071 pp., 297 figs., 1903. 
Zimmermann, A. Botanical Microtechnique. (Transl. by J. E. 

Humphrey.) 296 pp., 63 figs., 1893. 

Texts. Barnes, Czapek, Deherain, Green, Jost, MacDougal, 
Pfeffer, Stevens. 



CHAPTER XII 

RESPIRATION, AERATION, AND FERMEN- 
TATION 

The importance of air in the maintenance of animal 
efficiency has been recognized as long as mankind has 
existed. Shortly after the discovery of oxygen by La- 
voisier and Priestly (1774), Scheele showed that air exhaled 
by animals contains a smaller proportion of oxygen and 
an increased content of carbon dioxid. That marks the 
beginning of our knowledge of one of the products of 
respiration and of the extensive use of oxygen (0 2 ) by 
active living things. It was not, however, until later that 
the relation of plants to oxygen was understood ; and at 
that time, of course, the fundamental nature of the cata- 
bolic or destructive changes taking place with or without 
free oxygen in both animal and plant cells could not be 
suspected. 

160. The term " respiration." — Long before there was 
any accurate knowledge respecting the nature of the chem- 
ical changes (whether anabolic or catabolic) which may 
proceed in the cell, the term " respiration " was in use to 
denote in animals " breathing," — this latter term never 
aptly applying to any part of the process in plants. 
With the progress of physiological study upon plants and 
280 



Respiration, Aeration, and Fermentation 281 

animals the " heat of respiration " and the " energy of 
respiration " became well-known terms, so that respira- 
tion acquired a significance far wider than at first. In 
spite of this, the mechanism effecting gas exchange and 
the production of certain of the more common products 
of respiration (CO2 and water) long received consideration 
as respiration. In more recent times the determination 
that the fundamental changes involved are those taking 
place in the living cells themselves has therefore stretched 
further the use of this term. In both animals and 
plants these changes — alike in kind — are the essential 
features of respiration. 

At present there is a clear distinction between the 
usual oxygenating or aerating processes l and the en- 
ergy-releasing changes occurring in the cell. These last 
have been termed " energesis," — a term differing from 
catabolism chiefly in its more limited application and in 
laying emphasis upon the current view of the chief effect 
of respiration. 

161. An obvious result of respiration. — If an animal is 
given no food-materials, even for a single day, it must lose 
weight, although it may be supplied with the normal 
amount of water. A green plant placed in distilled water, 
and deprived of light — the essential condition for the 
making of organic food — will lose from day to day in 
dry-weight of its substance. Similarly, seeds germinated 
in darkness may, with requisite water, increase many times 
in bulk ; but the dry-weight will constantly decrease, — so 
rapidly, in fact, that quickly germinating seeds may lose 

1 The suggestion that these alone should be called respiration seems 
inadvisable. 



282 Plant Physiology 

in two weeks half their original substance, the expelled 
products being mostly C0 2 and H 2 0. 

This loss of organic substance is indicative of respiration, 
a process of catabolism and energy-release absolutely 
essential for the maintenance of every active cell, — plant 
or animal, — in both of which the essential process is 
substantially the same. 

162. The demonstration of respiration. — It is not at 
all requisite that respiration shall have exactly the same 
course in the cells of different organisms, nor is it necessary 
that it shall be dependent upon a common type of mechan- 
ism for the usually accompanying gas exchange. 

Once properly understood, activity and growth are 
themselves the best evidences of respiratory activity. 
Nevertheless, there is a gas exchange commonly inseparable 
from the process, and this gas exchange, since it is an 
accompaniment of respiration, serves as a simple and 
definite demonstration of the material end result, or of the 
type of material change which goes on. This gas exchange 
consists in the absorption and use of 2 and the elimination 
of C0 2 . Most important as a provisional experimental 
demonstration of respiration is the evolution of C0 2 . 
This is, of course, positive indication that organic matter 
is undergoing catabolic or dissimilatory transformations, 
and the C0 2 set free may be made, usually, a satisfactory 
qualitative or even quantitative measure of the com- 
parative rate of the process, especially in the higher plants, 
with which we are particularly concerned. 

Proof of 2 absorption and C0 2 production during the 
activity of living tissues is most definitely shown by an 
accurate analysis of the gases from a respiration chamber. 



Respiration, Aeration, and Fermentation 283 



This may not be practicable for the present purpose, and 
a simple suggestion of these gas relations is brought out 
by means of the often misused experi- 
ment with germinating seed. In this 
experiment the seed are placed in 
two bottles or jars carefully corked 
or sealed. After the lapse of a few 
hours a lighted taper lowered into 
one jar will be extinguished, and at 
the same time a thick film of barium 
carbonate will form on baryta water 
in a dish introduced into the other 
jar. (This film is far more pro- 
nounced than that which would form 
upon a similar test in a control jar 
containing air.) 

The above experiment suggests 
definitely two things : (1) that the 
content of C0 2 has been increased, 
and (2) that oxygen has disappeared ; 
but neither this nor any other sim- 
ple experiment, unfortunately, af- 
fords convincing proof that no FlG> 69 . simple respiro- 
other change takes place. scopes. 

A far better demonstration of the evolution of carbon 
dioxid is obtained by employing growing seeds in other 
types of apparatus, two of which may be briefly referred 
to : (1) by germinating seeds in a bottle or test-tube over 
baryta water (Fig. 69), or in a chamber in which a dish 
of baryta water is placed, and by comparing the amount 
of the precipitate in this respiration chamber with that in 




284 Plant Physiology 

a control vessel lacking seed ; (2) by drawing air deprived 
of C0 2 through the respiration chamber, and catching 
the C0 2 given off in a wash-bottle of baryta water. For 
quantitative work it is absolutely necessary to study stand- 
ard methods of C0 2 determination and to employ these 
with all the chemical precautions required ; especially 
important is a gravimetric method in which the C0 2 is 
caught in potash bulbs. 

The rapid inhibition of growth in the absence of oxygen 
may be taken as indicative of the use of this atmospheric 
constituent. The effect upon growth of an atmosphere 
deprived of oxygen may be demonstrated by a compara- 
tively simple experiment in which germinating seeds are 
placed in a tube containing normal air, and this is compared 
with another from which the 2 (and C0 2 ) are absorbed, as 
explained in the laboratory instructions. 

163. Respiratory phenomena in aerobic respiration. — In 
the type of respiration thus far particularly considered 
oxygen has free access to the respiring cells, and it is used 
in promoting chemical changes. This is called aerobic 
respiration, as distinguished from anaerobic respiration 
(subsequently discussed), which proceeds in the absence 
of free oxygen. 

The following may be given as a concise summary of 
the aerobic respiratory phenomena including the accom- 
panying gas exchange in green plants : — 

(1) Along with other gases, oxygen diffuses into the 
tissues, it is absorbed by the cell-sap, and it reaches all 
parts of the protoplasm of the cell. 

(2) Oxygen promotes catabolic processes, and whether 
through the protoplasm and its constituents directly, or 



Respiration, Aeration, and Fermentation 285 

chiefly through foods associated with the protoplasm, it 
takes an active part in the chemical changes of the cell, 
as a result of which the ultimate excrete products C0 2 and 
H 2 may be formed. 

(3) Complex organic molecules are decomposed; thus 
simpler products are produced, and kinetic energy is 
released. A part of this energy is in some manner utilized 
by the plant in growth and other activities, while another 
part, set free as heat, has little or no obvious value. 

(4) Of the final excrete products of this series of changes, 
the most significant is carbon dioxid, and it is eliminated 
from the tissues by the general diffusion mechanism in- 
volved in the entrance of gases. 

164. Oxygen promotes catabolic processes. — The 
studies which have been made upon the decomposition 
and hydrolysis of protein and other foods and the iden- 
tification of oxidizing and hydrolyzing enzymes as 
of widespread occurrence in cells point clearly to decom- 
position as the essential nature of the process. By some 
the decomposition of the protoplasm (cf . Barnes and others) 
is regarded as most important, while others view the pro- 
cess as essentially, or at least in part, a decomposition of 
foods, including carbohydrates and fats. Neither view is 
at present entirely satisfactory, but it is not possible here 
to review the evidence. It is certain, however, that the 
presence of free oxygen involves ultimately the decom- 
position of less material and a more economic energy- 
release. Moreover, normal respiration, and consequently 
the growth of most plants, is promptly checked by even 
the temporary exclusion of free oxygen. 

165. The ratio of 2 absorption to C0 2 production. — By 



286 Plant Physiology 

many physiologists respiration has long been considered 
to be a combustion process. If by combustion one under- 
stands a direct union of the 2 with C in such a manner 
that C0 2 is a direct result, this comparison is unfortunate. 
The combustion of any product results in a perfectly 
definite amount of energy as heat ; and this heat-energy 
may be very simply determined, whether it involves the 
combustion of coal, of proteins, of starch, or of cane- 
sugar. There is, furthermore, in combustion (however 
produced) a definite relation between the amount of oxy- 
gen needed and the amount of carbon dioxid given off. 
There is therefore a definite C0 2 / 2 ratio ; thus the 
combustion of glucose w r ould require 6 molecules of 2 , 
and 6 molecules of C0 2 would be produced, by the follow- 
ing formula : — 

C G H 12 6 + 6 2 = 6 C0 2 "+ 6 H 2 0. 

The combustion quotient in this case is unity. In respi- 
ration the transformations are not necessarily complete, 
and the respiratory quotient is seldom exactly unity, and 
as a consequence there are by-products and products 
less stable then C0 2 . The quotient is affected by tem- 
perature and other environmental conditions ; thus it is 
possible to picture a more complex and less definite se- 
quence.of changes in which protoplasm is involved. The 
series of transformations may be of the same type in the 
two cases, but they are not properly regarded as compa- 
rable processes so far as may be determined at present. 
166. Respiratory activity. — Respiratory activity is 
greatest during periods of rapid growth and differentiation. 
So soon as adequate water is absorbed by seeds previously 



Respiration, Aeration, and Fermentation 287 

dried, active respiration begins. The curve of C0 2 ex- 
cretion depends upon various external conditions, but a 
maximum is generally obtained by the time shoot and root 
are fairly elongated, after which there may be a rapid or 
gradual decline until the seedling is well developed. The 
curve shown (Fig. 70) is made up from data given by 
Mayer, and it shows the results with seedlings of wheat 
at a temperature of 23.8° C. 

This curve may be taken as merely a sample of the res- 
piratory rate during germination, although the curves for 
other seed may not be closely conformable. Frequently the 
rate of respiration for germinating seeds is, with respect to 
weight, about equivalent to that of man, generally given 
as about 1 per cent of the body weight for 24 hours. Seeds 
which germinate rapidly may, however, lose, under favor- 
able conditions, one third of their dry-weight during a 
period of 10 days, which is an average of about 3 per cent 
per day. In such cases, therefore, the intensity of respira- 
tion is relatively greater than for many warm-blooded 
animals. 

As the embryonic tissues in the plant are relatively 
reduced, the respiratory ratio will fall, so that well-de- 
veloped plants, or those growing slowly, will show, with 
respect to body weight, an exceedingly low ratio. Plants 
or organs passing into a resting stage will fall to a minimum 
which, in the case of dry seeds and well-protected bulbs, 
may approximate zero. On the other hand, opening 
flower-buds may show a high respiratory activity, often 
relatively greater than at the time of germination. 
According to Pfeffer, rapidly growing bacteria may con- 
sume oxygen at a rate 200 times as rapid as required by 



288 



Plant Physiology 




Respiration, Aeration, and Fermentation 289 



Normal tubers 
xi. 10, '95 
9 : 45-10 : 45 a.m. 




290 Plant Physiology 

man, and this fact is significant with respect to the im- 
portance of these organisms in the general disintegration 
of organic materials. 

167. Respiration of wounded plants. — It has been re- 
peatedly demonstrated that the respiratory activity is 
increased by injuries to the tissues. Precise data upon this 
point have been contributed by Richards. He employed 
chiefly potato tubers, but the experiments with these tis- 
sues were supplemented by carrot roots, also certain seed- 
lings, leaves, and willow-twigs. In general, it is found 
that following injury there is increased respiration for a 
time. Usually after two days — under the conditions of 
the experiments — the activity again declines to a rate 
more nearly the normal. The chart (Fig. 71) shows the 
CO2 developed from the respiration of 24 small potato 
tubers (weighing 200 grams) before and after wounding, 
the wounding consisting of slicing the potatoes length- 
wise. 

The ordinates represent milligrams of C0 2 , and the 
abscissae, time intervals of one hour each, on parts of 
several succeeding days as indicated. The sudden rise 
in C0 2 production after the injury of such solid tissues is 
explained by the inclusion of C0 2 , which is then rapidly 
lost during the first two or three hours. 

168. Heat release. — Exact determinations of the heat 
release in plant respiration have not been possible with 
the experiments as generally conducted. It has long 
been shown, however, that the temperature inside of the 
respiration chamber containing germinating seeds, open- 
ing flower-buds, and other materials exhibiting vigorous 
respiration maybe 5 or 10° C. above that of the surrounding 



Respiration, Aeration, and Fermentation 291 



air. In rough experiments, how- 
ever, much of the heat is lost by 
radiation, and such data are merely 
suggestive. Complex calorimeters 
are in use to measure the heat of 
animal respiration, and recently an 
advance has been made in simple 
plant experimentation by the use 
of the doubled-walled silvered 
Dewar bulbs, or thermal bottles. 
In preliminary experiments with- 
out sterilization precautions, Peirce 
secured a rise of temperature with 
an unweighed quantity of peas (in 
a silvered 250 cc. Dewar flask) 
amounting to about 25° C. in 
three days, with a maxi- 
mum rise of about 40° C. 
reached on the seventh 
day. With sterilization 
precautions and employ- 
ing in 250 cc. silvered 
Dewar flasks, 300 seeds 
of Canada field peas, a 
maximum difference of 
about 20° C. has been 
observed in some 
class tests. 
This does not 
represent the 
actual differ- 




Fig. 72. Dewar bulb. 



292 



Plant Physiology 




Fjg. 73. Dewar bulb in cross-sec- considerable 
tion, showing also method of ex- 
periment, — heat release. 



ence, since heat is lost even 
from the silvered flasks. It 
is greatly to be desired that 
an accurate calorimeter may 
be perfected. 

169. The mechanism for 
gas exchange. — To a con- 
siderable extent, the histo- 
logical mechanism permit- 
ting the diffusion of oxygen 
through the tissues of the 
plant is the same as for 
carbon dioxid used in photo- 
synthesis. Nevertheless, 
the cells active in photo- 
synthesis are localized, in 
higher plants, and the 
mechanism which permits 
aeration, in general, is 
more complex and may be 
treated. 

Since respiration is pro- 
ceeding in every living cell, 
the possibility of a fairly 
rapid gas diffusion must ex- 
tend to the remotest tissues. 
In developing or mature 
roots, stems, leaves, and 
fruits, there may be found 
intercellular 
space providing for diffusion 



Respiration, Aeration, and Fermentation 293 




^^^^§IC2^^^ 



of gases; sometimes indefinite air chambers form in a 
variety of ways. The intercellular spaces are produced 
• by the splitting 
apart of the walls 
separating mer- 
istematic cells, 
generally at the 
angles. In this 
way, small 
spaces are pro- 
duced into which 
air diffuses, and 
when such spaces 
are numerous, 
they form practi- 
cally continuous 
air chambers. 
In some cases 
these spaces oc- 
cupy far more 
volume than the 
cells themselves. 
This is particu- 
larly true in the 
case of the mes- 
ophyll tissue of leaves. 

Very large air spaces are a characteristic feature of many 
water plants, and they are often accompanied by a pecul- 
iar distribution of the cells, or of irregular or stellate 
out-growths from these into the cavities at a later period, 
Again, during the process of growth, cavities may arise 




Fig. 74. Experiment suggesting the efficient 
aeration of the leaf. [After Detmer.] 



294 Plant Physiology 

through the more rapid growth of certain regions, and the 
rupture of cells adjacent, as in the case of certain grasses, 
and of many other plants generally possessing a loose 
central pith during the early stages of growth. In the 
more solid or woody stems, intercellular spaces constitute 
some part of the structure, and the better aerated cortex 
may be provided with lenticels, or special areas of loose 
tissue permitting gas exchange. These lenticels are 
interruptions of the more or less continuous corky enve- 
lope constituting an essential part of the true bark of 
many woody plants. 

Roots may, in general, obtain some oxygen in solution, 
but the cortical parts of these organs exhibit, as a rule, 
rather large intercellular spaces, so it is evident that this 
special type of diffusion mechanism for aeration is impor- 
tant likewise in subterranean organs. In fact, in many 
roots there may be found special tissues, apparently 
insuring a surplus of air, and such may be designated 
air-storage tissues. Certain plants inhabiting stagnant 
water are provided with special roots, or root branches, 
which seem to be important in aeration. To these organs 
the term "hydathodes" has been applied. 

The relation of leaf-stalk and blade to air or the con- 
tinuity of the aerating tissues may be very well empha- 
sized by the experiment shown in Figure 74, in which, 
when suction is applied to the tube, air passes through the 
leaf, and is given off in bubbles from the petioles below the 
surfaces of the water in the bottle. 

170. Anaerobic respiration. — It has been clearly 
demonstrated that respiration may proceed for a time, in 
most tissues and cells, when no free oxygen is available. 



Respiration, Aeration, and Fermentation 295 

Considerable diversity may be manifest as to the extent of 
this respiration, and in the case of germinating seeds, the 
nature of reserve foods is an important factor in this 
regard. In certain tissues, anaerobic respiration takes 
place to such an extent as to be very readily recognized 
by the usual demonstration of C0 2 production. Never- 
theless, while C0 2 is commonly an end product of this 
type of respiration, alcohol, lactic acid, hydrogen, and 
other products may be identified with it. Since no free 
oxygen is required, decomposition resulting in C0 2 and 
the other products mentioned are obviously by rearrange- 
ment of the atomic groups in the organic molecules. 
This type of respiration is therefore truly anaerobic, or 
without aeration. The term " intramolecular " has also 
been employed in this connection. 

Anaerobic respiration in the tissues of higher plants may 
be experimentally studied by use of germinating seeds, 
preferably starchy seeds, such as barley and buckwheat, 
but slices of potato or other solid tissues of this nature are 
also useful. The essential apparatus is the same as for 
aerobic respiration, but in this case it is necessary either 
to exhaust all air with a first-class air pump, or else to 
replace air with some gas which is physiologically inert. 
Hydrogen was formerly used as a gas of this nature, but 
as there is a possibility that it may be so injurious as to 
introduce error, nitrogen may be substituted therefor. 

Attention has been drawn already to the fact (section 
87) that anaerobic respiration of roots is found to result 
in the production of traces of some excrete organic acids, 
whereas, in aerobic respiration, C0 2 alone is evolved. All 
of the results point to the conclusion, therefore, that by- 



296 Plant Physiology 

products and wastes are far more abundant in anaerobic 
respiration, owing to the lack of oxygen to stimulate the 
changes resulting in the production of C0 2 and water. 
With respect to micro-organisms which frequently demon- 
strate anaerobic respiration, this subject is further dis- 
cussed in succeeding sections upon fermentation. 

171. Fermentation. — The original significance of this 
term had reference to decomposition or change in or- 
ganic substances, such as sugar solutions and cider, ac- 
companied by the evolution of bubbles of gas, and 
generally by the production of alcohol. After the work 
of Pasteur and others, it was apparent that such changes 
(although they may be due ultimately to enzyme action) 
are in consequence of the activity of micro-organisms, 
which are then defined as having the capacity to ferment 
certain substances. 

In more recent times the term has applied to a variety 
of types of decomposition due to microscopic organisms, 
and also to the action of a large class of enzymes (soluble 
ferments), whether obtained from micro-organisms or 
from higher plants. The " ferment " action of the great 
majority of enzymes does not involve the liberation of C0 2 . 

In general, fermentation phenomena may be regarded 
as representing the various steps in decay. The type of 
fermentation in any particular case is usually given the 
name of the chief product produced, although in some cases 
it is the group of substances acted upon which stands 
sponsor for the name. In this chapter, only a few of those 
fermentation phenomena are discussed that are induced 
by the growth of micro-organisms, and as a result of which 
alcohol and certain acids are produced. 



Respiration, Aeration, and Fermentation 297 

172. Lactic fermentation. — If no precautions are 
taken to prevent the contamination of milk as it is drawn 
from the udder, it normal^ undergoes lactic fermentation, 
at the temperature of the dairy or living room. Among 
the organisms usually finding access to the milk, are 
bacteria (especially Bacterium lactis acidi) the action of 
which produces a slight acidity or souring, and later a 
marked effect exhibiting itself in the precipitation of the 
casein (curdling). The acid developed is largely lactic, 
and the course of the main changes referred to, involving 
the milk sugar (lactose, which is first converted into 
hexose sugars), is probably substantially thus : — ' 

C 12 H 22 O n + H 2 = C fi H 12 6 + C 6 Hi 2 6 = 4(C 3 H 6 3 ) 

lactose glucose galactose lactic acid 

One or more of a variety of organisms produce this end 
result. In general, they utilize as food a portion of the 
sugar, and they may produce, in small quantities, beside 
lactic acid and carbon dioxid, the following : one or more 
of several organic acids, also hydrogen, nitrogen, and traces 
of methane, depending somewhat upon environmental 
conditions. The production of lactic acid is so rapid in 
milk that the medium is soon sterilized with respect to the 
presence of other organisms ; but this is scarcely an ad- 
vantage to the lactic forms, since this fermentation and the 
activity of the lactic organism is usually brought to a 
standstill when about .8 per cent of acid has been pro- 
duced. According to certain investigators, a small amount 
of lactic acid as an excrete product may be produced 
during the anaerobic respiration of roots. 

173. Alcoholic fermentation. — Alcoholic fermentation 



298 Plant Physiology 

is more commonly brought about by the growth of various 
species of yeast in liquids or moist substrata containing 
certain sugars. Some organisms show a marked specific 
election with respect to the sugar fermented ; but in 
general, the hexose forms are most important, while trioses 
and nonoses are sometimes used. When abundant 2 
is supplied, the yeasts may grow rapidly, utilizing the 
sugars as foods, and effecting relatively little fermenta- 
tion. In the absence of sufficient 2 for rapid growth 
the organisms grow slowly, but fermentation proceeds 
vigorously. The decomposition of glucose yields ethyl 
alcohol and carbon dioxid as chief products, expressed 
conveniently by the following equation : — 

C*Hi20 6 = 2(C 2 HeO)+C0 2 

glucose ethyl alcohol 

On account of this capacity to produce alcohol, yeasts 
are the important organisms utilized in the production of 
alcoholic beverages, and the proper regulation of growth 
and fermentation is the essential factor in economic pro- 
duction. Starch, cane-sugar, and other carbohydrates 
transformable into the fermentable types are, of course, 
ultimately used, if first hydrolyzed, as in the malting 
process. 

Yeasts are unusually resistant to the alcohol produced, 
but the fermentative activity declines rapidly at 12 per 
cent, and at 14 per cent there is usually complete inhibi- 
tion. This concentration, however, is far greater than 
that which the more resistant molds may endure, com- 
monly from 4 to 5 per cent. It is well known that the pro- 
duction of C0 2 bubbling through and held by the cohesive 



Respiration, Aeration, and Fermentation 299 

gluten of wheat flour, is the important factor in light- 
bread making. It is of interest to note that anaerobic 
respiration in higher plants results in the production of a 
small amount of alcohol, so that the two processes are 
comparable. 

174. Acetic fermentation. — Acetic fermentation in 
nature generally follows the alcoholic, and it is brought 
about by the so-called acetic bacteria. These organisms 
effect the oxidation of ethyl alcohol in weak solution to 
acetic acid, probably in two steps, with the following, 
general result : — 

C 2 H 5 OH + 2 = CH3COOH + H 2 

ethyl alcohol acetic acid 

At the same time some alcohol is utilized and C0 2 pro- 
duced. In commercial vinegar-making, cider, weak wine, 
and other products of this nature are utilized, and there is 
a slow and a quick method of procedure depending upon 
1 the aeration. 1 

LABORATORY WORK 

Loss of weight. — Select two lots, each of 25 seed, of peas or beans. 
Determine the dry-weight of one lot and record. Soak and ger- 
minate the other lot in the dark closet upon a plate containing 
moist filter paper. When the seedlings have grown about as 
long as they will from the food material of the seed, determine 
the dry-weight and compare with those ungerminated. 

Absorption of 2 and evolution of CO2. — Into jars or wide 
mouth bottles put some soaked or germinating seeds of peas or 
beans, and cork tightly or seal. After a few hours, or next day, 

1 Prescott, S. C, "Wine, Cider, and Vinegar," Bailey's Cyclopedia of 
American Agriculture, 2 : 181-186. 



300 Plant Physiology 

test the air in one with a lighted wax taper and in another 
with a small dish of baryta water, comparing with similar 
tests carried out in a bottle containing only air. Explain the 
results and indicate the limitations of the experiment. 

Evolution of CO- 2 . — Pour some baryta water into a large test- 
tube, then introduce a closely fitting perforated cork upon which 
two or three germinating seeds may be placed ; cork and seal 
(Fig. 69). Set up a control experiment without the. seeds ; in a 
few hours compare and discuss the results with respect to the 
baryta water. 

Set up an apparatus as suggested in section 162 consisting of 
a chamber with germinating seed, or other favorable material, 
connected on the side toward the inflow of air with two wash 
bottles of potassium hydrate, to take out the C0 2 of the air, 
and on the side toward an aspirator or filter pump with two 
bottles of baryta water, ! for the demonstration of any C0 2 
given off. Before connecting up with the baryta water bottle 
draw air through the apparatus to remove the normal air. 
Connect with the baryta bottle, darken the respiratory chamber 
[Why ?], and draw air through the entire series for an hour or 
two. Describe the result. 

In quantitative work a standard method of CO2 determination 
should be employed, preferably a gravimetric method, in which 
potash bulbs are used, protected as to acquisition and loss of 
water by calcium chlorid drying tubes. In order to demonstrate 
anaerobic respiration in seed or other material the most satis- 
factory simple method is to replace the air in the apparatus with 
hydrogen. For this purpose a hydrogen generator is required 
in connection with the other apparatus described. There is 
evidence, however, that nitrogen is preferable to hydrogen, but 
in most laboratories it is not practicable to employ this gas. 

Heat release during respiration. — Soak for about 12 hours 

1 Make a concentrated solution of barium hydrate, with excess of the 
barium salt, keep the bottle or flask well stoppered, and decant off or 
pipette out the liquid, when needed, avoiding all unnecessary exposure 
to the air. 



Respiration, Aeration, and Fermentation 301 





somewhat more than 200 grams of 
field peas or wheat, and after soaking 
weigh out two lots of 100 grams each. 
Kill one lot by immersing it in boil- 
ing water for about 10 minutes, then 
place both the killed and the living 
seed in separate cheesecloth, bags and 
immerse each in formalin solution 
(1 part to 600 parts of water) for 
15 minutes. Take the bags from the 
formalin and dip them into boiled 
water with as little handling as pos- 
sible (the water should be at room 
temperature or at the temperature of 
the incubator to be used). Have 
thoroughly cleaned by the chromic 
acid mixture two double-walled, vac- 
uum, silvered flasks or Dewar bulbs 
of 250 cc. capacity (Figs. 72 and 73). 
Sterilize these also by rinsing with 
the formalin solution above indicated. 
Provide each flask with a short vial 
or small dish of KOH protected from 
tipping over by a wire cage lowered 
by a string, also previously steri- 
lized as to the exterior. Pour care- 
fully each lot of seed into a separate 
Dewar bulb, insert a standard ther- 
mometer, previously dipped in forma- 
lin and rinsed, and plug the flasks 
with wool. Wrap the flasks well with 
felt or woolen cloths and place them 
at a temperature as constant as pos- 
sible. Ten minutes later take the 
temperature of each, and at intervals 
of 12 hours for several days, or until 
there is a rise of temperature in the Fig. 75.^0xygen and growth. 




302 Plant Physiology 

control flask (containing killed seed) which would indicate con- 
tamination, necessitating the close of the experiment. Plot 
curves of the temperature conditions in the two flasks. 

Oxygen and growth. — After introducing seeds (Fig. 75) 
attached to a cork into A (shown at the left), fill AB with 
water recently boiled and recooled. Close stop-cock N, invert, 
connect with C (a tube with air only) and with D (a tube con- 
taining potassium pyrogallate (see p. 221). Shake the series 
(stopcock O open) until the oxygen is absorbed, close O, im- 
merse vertically the OD end of the series into boiled, cool 
water, disconnecting D, and opening O temporarily, to relieve 
(by inflow of water) the negative tension due to the absorption 
of oxygen. Close O, remove the apparatus from the water, 
open N, permitting the oxygen-free air to escape into A 
displacing water. Then close N, detach the parts below the 
latter, and place the AB part (as shown at the right) under 
temperature conditions favorable for growth. Compare at 
intervals the growth with seeds similarly placed in an open tube. 

Fermentation of sugars. — Properly fill two fermentation tubes 
(preferably the Kiihne form) with a 10 per cent sugar solution 
(fresh) of each of the following : glucose, sucrose, and lactose. 
In each tube insert a fragment of pressed yeast, plug the mouth 
lightly with cotton, and in 24 hours or more compare the pro- 
duction of gas caught in the closed arm. Insert in those showing 
gas a stick of caustic potash and explain the results. 

For experiments such as the above, continued so short a time, 
it is unnecessary to supply mineral nutrients, nor are sterilization 
precautions necessary. 

Alcoholic fermentation. — Prepare 500 cc. of a modified Pasteur 
solution to contain the following : — ■ 

Glucose 75 grams 

Ammonium tartrate 5 grams 

Potassium di-hydrogen phosphate 1 gram 

Calcium chlorid 5 gram 

Magnesium sulfate 5 gram 

Water 500 cc. 



Respiration, Aeration, and Fermentation 303 

Pour this into a 1-liter Erlenmeyer flask and add 2 grams of 
pressed yeast. Fit the flask with a cork through which passes 
the short arm of a piece of glass tubing bent so that the long 
arm may reach over through the cork of a wash bottle containing 
baryta water. Do not connect with the baryta water, however, 
until there is time for the air in the flask to have been driven 
out by the gas which is being produced. (How may this be 
determined, approximately?) Describe the result. 

When the fermentation is practically complete, or after one 
week, the flask containing the fermented solution may be con- 
nected with a condenser and distilled. At a temperature of from 
80-85° C. redistill, and when a few cc. have been caught note the 
odor ; then pour into a test-tube, add a crystal of iodine, heat 
gently to 60° C, and maintain at this temperature while adding 
a strong solution of caustic soda until the iodine dissolves. A 
yellow precipitate of iodoform is indicative of the presence of 
alcohol. 



References 

Barnes, C. R. The Theory of Respiration. Bot. Gaz. 39 : 

81-98, 1905. 
Blackman, F. F. Optima and Limiting Factors. Ann. of Bot. 

19: 281-295, 2 figs., 1905. 
Bttchner. Ber. d. deut. chem. Gesellsch. 30:117-124,1896. 
Fuhrmann, F. Vorlesungen iiber Bakterienenzyme. 136 pp., 

9 figs., 1907. 
Klocker, A. Fermentation Organisms. (Transl. by G. E. 

Allan and J. H. Millar.) 392 pp. 
Kostytschew, S. Ueber die norm. u. die anaerob. Atmung bei 

Abwesenheit von Zucker. Jahrb. f . wiss. Bot. 40 : 563-592, 

1904. [See also Ber. d. d. bot. Ges. 24 : 436-441. 
Peirce, G. J. A New Respiration Colorimeter. Bot. Gaz. 

46 : 193-202. 
Puriewitz. Physiolog. Unters. iiber Pflanzenatmung. Jahrb. 

f. wiss. Bot. 35 : 573-610, 1900. 



304 Plant Physiology 

Richards, H. M. The Respiration of Wounded Plants. Ann. 

of Bot. 10 : 530-582, 2 figs., 1896. 
Stoklasa, J. and Cerny. Isolierung des die anaerob. Atmung 

der Zelle d. hoh. org. Pflanze u. Tiere Bewirk. Enzyme. Ber. 

d. deut. chem. Gesellsch. 36 : 622-634, 1903. 

Texts. Barnes, Detmer, Ganong, Jost, Pfeffer. 



CHAPTER XIII 
GROWTH 

A proper conception of growth and important growth 
relations is fundamental in plant production. Growth 
necessarily receives consideration at least indirectly 
throughout every chapter, for it enters into any discussion 
of the relation of the plant to factors of environment ; to 
the making, use, and accumulation of food-materials ; 
and to the phenomenon of reproduction as well. In gen- 
eral, the practical measure of growth is yield. It is im- 
portant, however, to examine somewhat more carefully 
certain observations and fundamental facts regarding the 
mechanism of growth. 

175. The factors. — Growth is conditioned by internal 
and external factors. Among the internal factors must 
be assumed vitality, not explainable, yet known as an 
attribute of the living mechanism ; heredity, operating to 
reproduce specific form ; and often a certain food-supply. 
The external factors are many of the environmental con- 
ditions previously enumerated (section 5) ; and essential 
are moisture, a certain range of temperature, a source of 
oxygen, the several nutrients and crude food-materials, 
and (for continued growth in green plants) light. These 
factors in relation to growth and development receive 
special consideration as independent topics. 
x 305 



306 



Plant Physiology 



Klebs has in recent years developed important relations 
between the continuance of growth and certain external 
factors. For a few plants he has indicated the conditions 
tending to maintain vegetative growth and he has con- 




Fig. 76. Effect of conditions on the growth of pine needles : the short 
needles were produced during the season of transplanting (poor water- 
supply). 

trasted these with the influences inducing flowering — 
tending toward maturity. These are subsequently re- 
ferred to (section 225), but it is important here to note 
that most plants exhibit such complex relations as to 
render the problem especially difficult. 



Growth 



307 



176. Evidences of growth. — Observed as a whole the 
growth of any crop from seed-sowing to harvest is an 
obvious phenomenon. In general, the popular concep- 
tion of growth in flowering plants is that conspicuous 




Fig. 77. The effect of complex factors on the growth of corn. 

form of increase in size and weight which may be noted 
as the seedling develops into the mature plant, as the 
rapid exfoliation of leaves, or as the unfolding of the 
flower-cluster. Growth is associated with the formation 
and extension of living cells, and it may result in pro- 
nounced changes in external form or in internal structure. 
Growth involves at least two distinct phases. The one 
is increase in length, and often in size — extension ; the 



308 



Plant Physiology 



other is a change in internal structure, either within the 
cell, or affecting groups of cells, resulting in differentiation. 
Extension is evident, and differentiation may be obscure ; 
when the flower is fully open, for example, growth processes 
may go on within, which may or may not result in evident 
increase in size or weight, but new and important structures 
may be formed, and there is growth. Just so there is no 




Fig. 78. 



A potato sprouting in a dry, hot atmosphere, and in strong 
light. 



increase in the size of the incubating hen's egg, but by 
growth the little chick is soon developed from the simple 
fertile egg cell, with its stored food-material. 

177. Growth of the embryo. — Growth of the embrj'o 
(Fig. 79) in plants has been nowhere more carefully studied 
than in certain of the crucifers, notably in Capsella Bursa- 
Pastoris, a classical example among dicotyledons, which 



Growth 



309 



also may be taken as a type. After fertilization there is 
developed a row or filament of cells, called the proembryo, 
invested at the be- 
ginning, as a rule, 
with the endosperm. 
The apical cell of the 
proembryo divides 
longitudinally, and 
there are then cross 
and longitudinal divi- 
sions, either occurring 
first, thus producing 
a stage with 8 cells of 
the embryo proper, 
the remaining fila- 
mentous portion being 
designated suspensor. 
The following com- 
plete description of 
the embryonic growth 
from Coulter and 
Chamberlain l i n d i- 
cates the differentia- 
tion of the important 
regions of the young 
plant. 

" Whether the 
transverse division precedes or follows the second 
longitudinal division, it separates the cotyledonary and 

Coulter and Chamberlain, "Morphology of Angiosperms, " pp. 
196-198. 




Fig. 79. Germination of zygote and em- 
bryology of Lepidium : fertilized egg (A) ; 
proembryo (B) with suspensor (S) and 
apical embryo cell (e) ; later stages (C, D, 
E, F) showing development of stem, root, 
and cotyledonary parts. [After Curtis.] 



310 Plant Physiology 

hypocotyledonary regions of the embryo. In the octant 
stage the dermatogen begins to be differentiated, the 
periclinal divisions appearing first in the terminal oc- 
tants and proceeding toward the root end of the embryo. 
The differentiation, however, is almost simultaneous, so 
that the dermatogen is soon completed, except that of the 
root-tip, which is derived from the adjacent cell of the 
suspensor, and appears comparatively late. The periblem 
and plerome are differentiated early from the tissue within 
the dermatogen. The stem-tip and cotyledons are de- 
rived from the four apical octants, and the bulk of the 
hypocotyl from the four basal octants. The root-tip, 
however, is completed by the adjacent cell of the suspensor. 
This cell divides transversely, the basal daughter-cell 
taking no part in the formation of the embryo, but the 
other daughter-cell (hypophysis of Hanstein) filling out 
the periblem and dermatogen of the root-tip. The 
hypophysis divides transversely, the daughter-cell next 
the embryo completing the periblem of the root. The 
other daughter-cell by two longitudinal divisions gives 
rise to a plate of four cells, each of which divides trans- 
versely, the plate of four cells toward the embryo com- 
pleting the dermatogen of the root-tip, and the other plate 
constituting the first layer of the root-cap." 

178. Polarity. — This term denotes a differentiation of 
the two poles of a growing cell or organ. Any part of 
any seed-plant or member is therefore recognized as hav- 
ing an apical pole and a basal pole, the apical being the 
direction of growth of the shoot, and the basal the direc- 
tion of growth of the root. From the preceding description 
of the growth and differentiation of the embryo the sig- 



Growth 311 

nificance of this phenomenon may be seen. Polarity is 
known merely through the behavior of organs. 

179. Elongation of roots. — The nature and distribu- 
tion of the tissues at the apex of the root axis have been 
noted. The elongation of the root may be readily followed 
by a simple experiment. The tip of a young seedling 
should be divided into zones by a half dozen or more 
• parallel marks about 2 to 3 mm. (about to inch) apart, the 
first mark, however, being practically at the tip. The 
marks can be made with India ink and a pen, or a very 
fine brush. 

Observations after from 6 to 24 hours, under favorable 
conditions, will show that the region of elongation is con- 
fined to a space covering usually only a few millimeters 
back of the tip. The zone farthest away may have 
already ceased to elongate, or practically so ; while those 
nearest the tip elongate at first slowly, then faster, to a 
maximum, after which they decline. In fact, if new zones 
were constantly marked off each would be seen to go 
through a certain grand period of growth. The impor- 
tance of the shortness of this region of growth with respect 
to the ability of the root to penetrate the soil has been 
pointed out in the discussion of the relation of the root to 
water and to the soil (section 30) . 

New lateral roots form in the quiescent region behind 
the root-tip ; such lateral roots originate in the pericycle, 
just within the endodermis, and as they push out they 
literally break through the cortical tissues, which latter 
are in part broken down and dissolved. Lateral roots are, 
therefore, said to arise endogenously. 

The growth and branching of the roots of agricultural 



312 Plant Physiology 

plants are in general increased by favorable food-supply and 
a water-content of the soil which shall not ordinarily be 
more than from 30 to 50 per cent of soil saturation, as 
discussed elsewhere. Under favorable conditions the 
rate of growth is rapid; for example, the roots of corn may 
elongate at the rate of about \\ inches per day. In many 
cases increase in size follows elongation, although it may 
be coincident with this. 

Increase in the size of roots commonly involves no 
shortening of the root-axis, yet in the rooting of certain 
bulbs, and following the germination of a few seed, shorten- 
ing may occur after the roots are fairly fixed in the soil, 
thus resulting in effectively burying or sinking the storage 
organ. The practical advantage is evident. 

180. The stem apex. — The stem apex of the flowering 
plant shows, like the root apex, no single apical cell from 
which growth proceeds ; instead, there is within the 
epidermis a group of cells rather indefinite in area which 
constitute the primary meristem. In the apical cells of 
this meristem divisions rapidly occur, and there is also a 
rapid extension of those somewhat older, or farther from 
the tip. This multiplication and elongation of cells is 
the direct cause of the observed increments of growth. 
The epidermal layer in order to accommodate this increase 
in growth is extended by divisions perpendicular to the 
surface (anticlinal divisions). 

The cells of the meristem are gradually differentiated 
posteriorly in two chief regions, — an outer, or periblem. 
and an inner, plerome. It is primarily within the plerome 
that vascular tissue in seed-plants is differentiated (sec- 
tion 186). 



Growth 313 

The rate of growth extension in seedlings or other small 
plants may be conveniently recorded by means of auxa- 
nometers of various forms. A desirable type of this in- 
strument produces a record by the following principle : 
A cord attached to the growing organ passes over a small 
wheel to a weight which takes up the slack induced by 
growth. The rotation of the small wheel carries with it a 
larger wheel from which a cord is connected with a pointer 
working against carbonized paper on a revolving drum. 
In this manner a continuous record is secured of the incre- 
ments of growth. 

181. The formation and exfoliation of leaves. — Origi- 
nating in the developing periblem just behind the apex the 
young leaves arise, in spiral order, or verticillately disposed 
about the stem. These are at first small protuberances 
resulting from cell divisions parallel to the surface (peri- 
clinal divisions) ; but in time they flatten and grow faster 
than the stem apex. They curve over the stem apex 
more or less to form a bud. 

In many annuals, and in perennial herbs, but only 
occasionally in trees, the leafy axis continues to elongate 
throughout the so-called growing season. If the shoot 
apex thus constantly elongates, each leaf in succession 
remains a part of the bud for only a short space of time, 
since by further growth, — more rapid on the upper sur- 
face, — complete exfoliation is soon effected. The exact 
point of origin of the leaf back of the stem apex depends 
in general upon the type of leaf arrangement in the 
species of plant. In the axil of each young leaf there 
is commonly formed later a region of growth destined to 
become a lateral bud. This bud also originates ordinarily 



314 Plant Physiology 

in the periblem in a manner very similar to the fundament 
of the leaf. 

182. The resting bud. — Aside from the method of more 
or less continuous leaf development in the bud which has 
been stated to be the rule during the growth period of 
annuals, it is necessary to consider more particularly this 
phenomenon with respect to certain trees. In a majority 
of trees the shoot axis is terminated during the summer or 
early autumn by a resting bud. This bud is merely a 
very short leaf axis protected by bud scales, the latter 
being structures homologous with leaves or leaf parts. 
As a matter of fact, this terminal bud may be more or less 
completely differentiated in the early portion of the 
summer, but it does not necessarily present the appear- 
ance of a resting bud until midsummer or later. 

Growth within the so-called resting bud proceeds very 
slowly, or may entirely cease, for several months during 
the winter. The following spring, with the return of 
favorable conditions for growth*, there is generally a rapid 
unfolding of the leafy shoot, or of the cluster of leaves or 
flowers. It must not be understood, however, that this 
resting bud cannot be forced into more or less immediate 
growth, or that it is necessarily formed so early in the 
season. As a matter of fact, it appears that adventitious 
buds may arise and develop shoots during a single season, 
and that water shoots may form no terminal bud until 
completing a season of growth more or less equivalent to 
that of an annual. 

183. Types of stem elongation. — Wholly apart from 
the exceptional cases referred to, the method of elongation 
of the shoot, or bud axis, in woody plants is diverse, and 



Growth 



315 



the several types given below should not be regarded as 
clearly distinguished one from another : — 

(1) Growth of the bud into a shoot may consist of the 




Fig. 80. Stages in the elongation of a shoot of pine. 

rapid elongation of the parts which have been previously 
laid down in the bud. In this case, the leaves which 
appear on the leafy shoot are merely the full number which 



316 Plant Physiology 

existed as leaves in minute form in the winter bud. Re- 
cent studies by Miss Moore indicate that a considerable 
number of our north temperate deciduous trees are of this 
type ; and a number of observations suggest that it is the 
method common among conifers. Excellent examples of 
this type are offered by the beech and pine. In the beech 
the first sign of activity in the spring is that of gradual swell- 
ing of the bud, and at first a rather general stretching of 
the internodes. The bud quickly doubles its former length, 
and by this time observations upon the method of elonga- 
tion are most easily made. It will be found that the 
growth increments in the basal internodes are at first 
stronger, successively passing to others, and the terminal 
internodes are the last to show rapid extension. Never- 
theless, there is a distinct grand period of growth for each 
internocle in turn. 

In the pine, on the other hand, there is this difference : 
the shoot is unsegmented, and every portion of the bud 
axis from base to apex becomes successively the region 
of greatest extension; although in this case extension is 
more nearly uniform throughout the whole shoot axis. 
It appears that pomaceous fruits ordinarily follow the 
plan of this general type, but peaches may frequently fall 
into the next class. 

(2) From the data available it would seem that the 
lilac, willow, and some other trees may develop normally 
during the summer a few more leaves than are ordinarily 
contained in the resting bud. In this case there is, of 
course, a formation of new nodes and internodes from the 
young meristem as the bud is expanded, or at least dur- 
ing the grand period of growth of the shoot. 



Growth 317 

It is quite possible that many trees show types of de- 
velopment from the resting bud dependent upon the con- 
ditions. In the Carolina poplar the writer has found that 
old trees may show an exfoliation of few if any more leaves 
than are normally contained in the bud, while younger 
trees in rapid growth may produce more than five times 
as many as were thus preformed. 

Frequently the leaves of the apple, pear, peach, and 
other fruits seem to be produced in clusters upon short 
branches or spurs; in those cases, as a rule, the internodes 
are suppressed, and the axis is therefore greatly shortened. 
This is particularly common on fruit spurs. It is impos- 
sible here to consider the modifications in growth accom- 
panying fruit production, the alternations of growth, 
elongation, fruiting, 1 etc. 

184. Fruit buds and age of shoot. — It is of importance 
to consider briefly the relation of fruit buds to the age of 
the wood in a few economic plants. In the grape, the 
fruit is borne on canes produced during the current year. 
Where a vine is left for a season unpruned, many buds 
upon each cane push into new shoots, yet relatively few of 
these will then bear fruit, or at least, large bunches of 
fruit. As a rule, the better fruit-producing canes on 
American grapes are developed from side canes (pruned 
to a bud or two) on a leader which is not less than two 
years old. 

Peaches and almonds develop fruit on wood which is 
one year old, and generally the fruit buds are more abun- 

1 For extensive observations on branches, fruit spurs, and other mat- 
ters of interest in this connection, the student should refer to Bailey's 
" Lessons with Plants," pp. 1-69. 



318 Plant Physiology 

dant toward the middle portion of the shoot. On the 
other hand, apples are produced on wood which is two 
years old, and the same is true of cherries and plums. 
The pear belongs, in general, to the apple class, but on 
old spurs the relations may be somewhat more complex. 
It is evident, therefore, that the pruning practices with 
respect to any particular fruit must take into consideration 
not merely the effects upon general growth and form of the 
tree, but must consider also a proper regulation of the 
buds, shoots, or branches which are to produce fruit. 

185. Persistence of the rest period in temperate 
regions. — Owing to the periodic production of the ter- 
minal bud, the shedding of the leaves, and the passage 
of the plant into a definite state of rest each autumn, 
it has been more or less assumed that this period is essen- 
tial, and that it may only with great difficulty be short- 
ened. It has been felt, particularly, that it is extremely 
difficult to force perennials into new growth before their 
normal resting period is completed. Klebs has shown, 
however, that under favorable conditions for growth, a 
number of plants require no winter rest period, and may 
be cultivated more or less continuously. 

More recently, Howard and others have demonstrated 
that, even with those plants which are most persistent 
in refusing to grow until the normal rest period is over, 
etherization or other special forcing may be effective in 
giving the necessary stimulus to early growth (as devel- 
oped later, section 197). First, however, it is necessary 
to indicate the effect of restoring, during the resting period, 
favorable conditions for growth. It is shown that of 
234 species of plants brought into the greenhouses at 



Growth 319 

Halle, Germany, from October 28 to November 4, more 
than one half, or 125 species, began to make growth 
promptly under greenhouse conditions. Thus it is 
apparent that a large number of deciduous trees merely 
require favorable conditions for growth in order greatly 
to reduce the normal rest period. 

Observation upon orchard and forest trees fully confirms 
this view. It happens frequently in temperate regions 
visited by drought in the early summer that the resting 
bud is formed early, and defoliation of many of the spring 
leaves may occur by midsummer. In such cases, a return 
of moist weather and favorable conditions in the late 
summer or early fall may result in a flush of growth from 
the resting bud of the same season. Sometimes this may 
be accompanied by fall blossoming. 

186. Differentiation of stem tissues. — A complete 
study of internal anatomy, or histology, is not the pur- 
pose of the following paragraphs. It is, however, essen- 
tial to note the common method of growth or develop- 
ment of some of the chief tissues and tissue systems 
within the plerome of the growing tip. There are pro- 
duced by division and differentiation of the tip meristem 
strands of elongated cells known as the procambium. 

In dicotyledons, these strands may be commonly 4 to 
10 ; typically they are disposed as an interrupted ring in 
the outer portion of the plerome, surrounded by cells of a 
less differentiated meristem, often termed the ground or 
fundamental tissue. The procambial strands become 
by differentiation the primary fibro vascular bundles. 
In developing the common type of bundle (collateral 
type), the inner portion of the procambium becomes the 



320 Plant Physiology 

xylem, or wood, in which wood-cells, pitted and spiral 
vessels occur. The outer portion develops the phloem, 
or sieve-tube and soft-bast part of the bundle. Between 
these two there remains a growing meristem, the cambium, 
which is most important in secondary growth, or thicken- 
ing, as subsequently stated. In woody plants this cam- 
bium becomes a continuous ring, being formed between the 
bundles by the differentiation of the ground meristem. 

In the root, however, the procambial areas originating 
the phloem and the xylem of the several bundles alternate 
radially (radial arrangement); and when a ring of cam- 
bium is formed, it is between phloem and xylem, as well 
as on the periphery of the xylem, thus constituting an 
irregular or lobulate ring. 

187. Secondary thickening. — Secondary vascular growth 
normally arises by the development of new bundles from 
the ring of cambium. Ultimately so many of these second- 
ary bundles may be interposed as to produce in woody 
or semi-woody plants a complete wood-cylinder. The 
bundles may be more or less completely separated one 
from the other by thin bands of ground-tissue, known as 
medullary rays. In perennials, the cambium differ- 
entiates each year (or season) new xylem within ; thus 
the cambium is carried farther and farther from the 
center. It develops new phloem without, so that both 
wood and bark are annually or seasonally augmented. 
However, the xylem vessels produced during the first 
flush of growth, that is, in the season of greatest activity, 
are larger and more important, and they have thinner 
walls than those produced later, so that a definite ring 
formation results. 



CfiOSS srcr/oAf 

fait weed 3 P r/„j? vsccJ ^ ^ /^W^ T J 




~6ast — ' rind (or cortex) 

10A/C/T UD/ NAL G£CT/OAi 

Fig. 81. Oakbranchin cross, tangential, and longitudinal section : cork 
(ck), cork cambium (cc), parenchyma (cor. p.), stereome (str), bast (bp), 
sieve-plate (sp), sieve-tube (st), duct (d), tracheid (tr), wood paren- 
chyma (wp), medullary ray (mr), and lenticel (I). [After Osterhout.] 



322 Plant Physiology 

In a monocotyledon, like the Indian corn, the procambial 
strands are rather numerous, and generally irregularly 
disposed. The differentiation of this tissue into the 
mature elements is also complete, so that there remains 
no growing tissue in the bundle, and there is no further 
growth by secondary thickening. The thickening in the 
stem which results between the young and mature stages 
of the corn is very largely due to an increase of the size of 
cells already laid down at an early stage. 

The leaves of many dicotyledons are generally more 
or less completely formed with respect to fundamental 
tissue very shortly after they begin to unfold, although 
there may be a subsequent growth and differentiation in 
the veins and veinlets. On the other hand, in the case of 
certain monocotyledons, especially, a growing zone, gen- 
erally indefinite in extent, may be maintained for a con- 
siderable time near the base of the leaf. A few plants, 
such as the ferns, also elongate for a time by growth at, or 
near, the apex. 

188. Growth of the cell. — It has been abundantly 
indicated that the growth of the organs of the plant, and 
of the plant as a whole, are dependent upon the capacity 
of the meristem or embryonic cells to extend or to divide. 
Extension is commonly associated with differentiation 
and maturity. It may result in a great relative reduc- 
tion of the protoplasmic content, or as previously shown, 
it may result in protoplasmic loss, and eventually in the 
death of the cell, the firm cell-wall alone remaining. 
This type of cell growth, therefore, usually produces a 
specialized tissue, and the differentiation is to some 
extent an immediate growth response, for the extent of 



Growth 



323 



these tissues may be determined, in considerable measure, 
by the conditions of growth. 

Many meristematic cells, especially cells of the pri- 




Fig. 82. Nuclear and cell division in the root of corn : cell with promi- 
nent resting nucleus (A) ; prophases of nuclear division, spirem (B) 
and chromosome (C) stages ; bipolar spindle (D) ; early (E) and late 
(F) anaphases ; telophases (G) and first evidence of cell-plate ; location 
of cell-wall clearly defined (H). [After Curtis.] 

mordial meristem, are so situated and conditioned that 
growth or increase of the protoplasmic content takes 
place, and at the same time the size of the cell may in- 
crease; this condition, however brought about, usually 
results in cell division. 



324 Plant Physiology 

189. Cell division. — Usually, in vegetative organs, 
division results in such manner that any meristematic 
cell, temporarily regarded as a primary (or often desig- 
nated parent) cell, produces two more or less equal sec- 
ondary (daughter) cells. Exceptionally, differentiation 
may accompany division; and, in any case, the subse- 
quent life history of the secondary cells may or may not 
be similar. It is obvious that cell division must carry 
with it the division of most of the essential organs of the 
cell. There is, in fact, division of nucleus, c3 r toplasm, 
and plastids. The nuclear division is of peculiar interest. 

190. Nuclear division. — The nuclei of both plants and 
animals seldom divide by a direct halving of the nuclear 
substance, or direct division. Such a type of division 
is, however, known. The usual process is complex, char- 
acterized by several distinct phases, all of which are 
apparentl} r important in securing the equal division of 
certain chromosomes, or nuclear segments, which appear 
during division. This indirect process is termed mitosis, 
or karyokinesis. The observation of nuclear division 
usually requires material which has been carefully fixed 
(with respect to protoplasmic structure), sectioned, and 
stained. 

A meristematic cell from the root of Indian corn may 
typify the usual phenomena (Fig. 82). During the 
growth of the cell, the nucleus exhibits toward certain 
stains definite reactions, and these are, for the most part, 
greatly intensified during division. The nuclear retic- 
ulum shows at first some small chromatic thickenings or 
scattered areas taking the stain more deeply, whilst the 
nucleolus is also deeply stained. When the reticulum- 



Growth 325 

like nature of the nuclear substance gradually gives way 
to elongate chromatic structures, or to a chromatic band, 
the prophase of the nuclear division is well advanced. 

Later there appear well-defined nuclear segments, 
termed chromosomes, these resulting apparently from the 
aggregation and growth of chromatic substance in a cer- 
tain area. Coincidently, the nucleolus is less stainable 
and may show an apparent degeneration, foretelling its 
final disappearance. The chromosomes thicken, the nu- 
clear membrane disappears, and out of the mass of fibrous 
protoplasmic elements now present, there is oriented first 
a multipolar, and later a bipolar, spindle with the chro- 
mosomes arranged as an equatorial plate. In this stage, 
the metaphase, spindle fibers are attached to either side 
of each chromosome, and a longitudinal split is apparent. 
The halves of each chromosome separate and the " daugh- 
ter " groups move (anaphase) to opposite poles of the 
cell, where the organization of the daughter nuclei pro- 
ceeds (telophase). Here a new reticulum is ultimately 
evolved and a nucleolus reappears, formed, doubtless, in 
some manner from the nuclear material. Upon the re- 
maining spindle fibers at the middle points thickenings 
occur, and these gradually extend as a plate between the 
two " daughter " cells. Thus the cell-space and the 
cytoplasm also are divided, and the cell division is com- 
plete. 

Upon the reappearance of the chromosomes in every 
successive mitotic vegetative division, the number of 
these segments is constant; that is to say, there is a defi- 
nite chromosome number for every species of plant, and 
the same is true of animals. 



326 Plant Physiology 

191. Cell division and respiration. — It is obvious that 
the growth of the embryonic cell in protoplasm often leads 
to a climax of energy-release in the complex activities of 
nuclear and cell division. When growth of the cell does 
not lead to division, or multiplication of kind, it is usually 
a progression towards differentiation, a process likewise 
involving abundant metabolic changes and energy-release. 
It is not strange that respiration in healthy organs is, in 
general, a measure of growth intensity. 

192. The relation of pruning to growth. — Pruning, as 
applied to trees, shrubs, and vines, is a practice which has 
as its chief ends a regulation of growth and fruiting, and 
a shaping or training of plants. Either one or the other 
of these ends may be purely incidental, but the process 
is most important as a thinning of the fruit buds, and for 
the regulation and distribution of new wood. The prac- 
tice must vary with the species of plant, and with the local 
ideas of proper size and shape. Properly performed, it is 
physiologically rational, and the world-wide development 
of the practice attests its effectiveness. Pruning should 
not be regarded usually as a special form of forcing for 
fruit production. 

In trees the leaf buds often develop most abundantly at 
the tips; that is, at the periphery of the entire tree, so that 
the tree grows as a constantly enlarging shell. There are 
many more buds produced on the periphery than could 
possibly be developed profitably. Ordinarily, many more 
begin to develop than could succeed. Pruning is needed 
to suppress some buds, and to permit others to grow more 
vigorously. It is also needed with certain fruits in order 
to cut out and restrict large branches, so that light may 



Growth 



327 



enter the central portion of the tree, for the encourage- 
ment of fruit production throughout. 

In the majority of cases, cutting out some of the old 
wood or pruning off a portion of the young wood, incites 
more vigorous growth in the parts remaining. A too 
heavy pruning may be distinctly injurious. It may incite 
a large sucker or water-shoot growth at the expense of 



■■'■.Sii^rtynTi W- ■-■• m •- - : ' ; * r-t- ' 



; KrV zSLj**? c^P 



Fig. 83. Pear trees trained against a wall. 

fruiting, produce a general weakening of the tree, espe- 
cially by loss of organic food to the roots, and finally 
become a source of danger through the unnecessary 
wounds. In general, pruning is most common in order to 
maintain a certain balance between vegetative growth and 
fruiting. No plant can illustrate this relationship better 
than the grape. A failure to prune during a single season 
will be followed by the development of a large number of 



328 



Plant Physiology 



canes, but the bunches of fruit will be small and poorly 
filled. 

Pruning at the time of transplanting is invariably 
necessary in order to keep the balance between root and 
shoot. Resetting or transplanting maj r result not only 
in injury to the roots, but often in the death of all rootlets ; 




Fig. 84. The healing of wounds, after cutting off a lateral branch ; first 
formation of callus (cl), after which three seasons (rings) of growth 
were required. [After Curtis.] 

and while the latter are being developed the leaf surface 
must be reduced. 

Ordinarily, pruning is a late winter practice, and this is 
desirable, in the first place, because there is no injury 
from bleeding, and secondly, on account of the prompt 
covering of wounds by growth in the spring. For the latter 
reason, also, branches are cut close to the main branch or 
stem, where practicable, and no large stubs are left. 

The covering or healing of wounds by the growth of 
tissues beginning about the margin of the wound is a 



Growth 



329 



response or adjustment bringing with it most important 
sanitary advantages. A wound long exposed is an almost 
certain beginning of a heart-rot of some type. The 
development of tissue covering the wound proceeds from 
the cambium. A callus or cushion of vigorous meriste- 
matic tissue is produced, and this is extended from all 
sides, until the wound is completely closed, when new 
wood-rings are laid down over it (Fig. 84). 

193. Budding and grafting. — The growth processes 
immediately involved in budding and grafting are well 
understood, but all of the relations of stock and scion are 
not so clearly defined. In both budding and grafting, 
the important principle is to unite the cambium of stock 
and scion. When held firmly in contact by grafting- wax 
or raffia, the meristems of the two individuals thus united 
develop a callus, effecting a 
close union ; and wood is 
subsequently laid down 
from each contributing part, 
cementing this union com- 
pletely. In general, a 
union so close as to insure 
the life of the scion is only 
possible when the plants 
are closely related. If 
stock and scion grow at a 
different rate, that is, if the 
seasonal rings of one are 
thicker than those of the 
other, there will be a con- _. 

; , , Fig. 85. Grafting : cambium of stock 

Siderable difference in size and of scion (on one side) in contact. 




330 Plant Physiology 

of trunk above and below the region of the graft, and 
this difference becomes more pronounced with age. Im- 
proper, or difficult, union, if it does not result in im- 
mediate death, will inhibit the transfer of material be- 
tween shoot and root, and may lead to an abnormal 
swelling in the region of the union. 

194. Scion propagation. — As referred to in the dis- 
cussion of reproduction, vegetative propagation is often 
desirable, and propagation by buds or scions possesses a 
variety of advantages, some of the most important of 
which are as follows : (1) for the maintenance of varietal 
characters, especially when the plant is of uncertain or 
hybrid origin, when a return to the seed would yield an 
unknown progeny, (2) for the more rapid propagation of 
desirable species and varieties, and (3) for certain advan- 
tages of growth or hardiness which may result by placing 
the scion on roots other than its own. 

195. Relation of stock to scion. — Commonly there is 
believed to be relatively little direct formative influence 
of the stock upon the scion, and an analysis of the facts 
thus far demonstrated makes it clear that, as a whole, the 
relations between stock and scion are very complex. The 
effect of the stock upon the total amount of growth is 
most evident in dwarf varieties, such varieties of the pear, 
for example, are obtained by grafting pear scions upon 
quince stocks. The scion is then furnished, in all proba- 
bility, by a root system less active in absorption, and the 
effects of this are evident not only in diminished size, but 
also in slight modifications of leaves and sometimes of 
fruit. Waugh has called attention to certain differences 
in vigor of growth as well as in size and serration of leaves 



Growth 331 

caused by the use of different stocks with plums. The 
Milton plum worked on Mariana stock is more vigorous, 




Fig. 86. Fasciated shoot of Fritillaria, apparently induced by rapid 
forcing. 

and develops larger leaves than when Wayland or ameri- 
cana stocks are employed. In the former case, however, 
the leaf serrations are finer. 



332 Plant Physiology 

In the case of some other fruits, greater hardiness or 
resistance to cold is secured by grafting upon hardy stocks. 
The sweet orange is now commonly grafted upon the 
rough lemon and upon the sour orange, both in Florida 
and California, although some believe that the quality of 
the orange is thereby somewhat affected. Several species 
of American grapes are notably resistant to Phylloxera 
(especially Vitis rotundifolia and V. riparia), and these 
vines are now commonly employed as stocks in certain 
sections of southern Europe where this insect has done 
great damage. Since the insect is mainly injurious upon 
the root, there is a direct advantage in using American 
stocks. 

The transmission of certain diseases, or pathological 
conditions, such as peach yellows, contagious chlorosis, 
etc., may occur by grafting, but in general it is felt that 
there is relatively little of what may be termed special 
chemical influence of the stock upon the scion. Litera- 
ture is full, however, of contradictions and strife regarding 
the mutual influence of stock and scion. An hereditary 
effect has been claimed, but the lack of definite w r ork with 
strains sufficiently pure, renders the whole matter proble- 
matical. 

196. Forcing. — This term is rather loosely employed. 
It may signify merely the production of plants out of 
season, generally under glass or other protection, such as 
the growing of tomatoes in the winter ; again, it may 
suggest the growing of plants which, in a particular lati- 
tude, require certain well-controlled conditions. These 
applications of the term require no further consideration 
physiologically. When, however, it is implied that fore- 



Growth 



333 




Fig. 87. Leaves of rhubarb grown under diverse conditions : in the 
open (A) ; forced in dark cellar after being frozen outside (B) ; forced 
in well -lighted cellar (C). 



334 Plant Physiology 

ing involves production under abnormal, or what may be 
termed intensified, conditions, that is, under conditions 
stimulating rapid growth, then factors may enter in 
which require special attention from the standpoint of 
growth-stimulation. High temperature, increased mois- 
ture, and an abundant food- supply are the factors com- 
monly involved in forcing. Under such conditions there is, 
of course, up to a certain maximum, a stimulation of vege- 
tation. High succulence and brittleness are characteris- 
tics of forced crops. 

For the production of salad crops, radishes, etc., forcing 
may be continuous, while in other cases, forcing condi- 
tions are employed to start resting plants or roots into 
rapid and vigorous growth for early market, as in the case 
of roots of rhubarb and asparagus brought in from the 
open. Large roots of rhubarb grown in the open for three 
of four years may be lifted in the late winter or early 
spring, reset in loose soil in a special cellar, hot bed, or 
greenhouse, and then forced into rapid leaf-stalk produc- 
tion. Forcing may also be employed for bulbs, tubers, 
and seed in the seed bed. The practice in general requires 
special care with ventilation ; it often demands subirriga- 
tion, and it repays a constant watchfulness with respect 
to sanitary surroundings. Otherwise, the conditions may 
greatly encourage the growth and spread of fungous 
diseases and the development of other pathological dis- 
turbances. 

A special phase of forcing has become important in 
recent times. This consists in awakening activity in 
dormant plants or organs by means of warm water or 
anaesthetics. 



Growth 335 

197. Etherization. — Etherization of plants and bulbs 
is rapidly becoming a common forcing practice with 
florists, and it is to some extent applicable in market gar- 
den work. By means of a suitable incubation in an atmos- 
phere of ether or chloroform, it is possible to furnish the 
incitation for rapid growth, particularly in the case of 
resting plants and dormant bulbs and roots. It is thus 
possible to bring such plants into more rapid vegetation 
and flowering, to meet the special demands of particular 
seasons or occasions. 

Stimulated by the many experiments of Johannsen, in 
northern Europe the practice has been very successfully 
employed in forcing lilacs for the cut-flower trade, while in 
southern Europe it is usually applied to the " mimosa," 
a species of Acacia. It is notably economical of time, 
space, and heat, in forcing many bulbous flowering plants. 

In general, a common method of etherization is as 
follows : The plants are exposed from 24 to 48 hours 
in a tight chamber or box to an atmosphere of ether vapor, 
with an ether tension preferably from 30 to 40 grams per 
100 liters of space (approximately -J- ounce per cubic foot). 
The concentration and the length of exposure should, 
however, vary to suit the material, the more delicate 
material requiring the weaker treatment. After treat- 
ment the plants are ordinarily placed immediately under 
conditions favorable for growth. 

If employed relatively early during the period of winter 
rest with the lilacs, marked contrast is shown between the 
forced and the control plants. During the early winter, 
in the latitude of New York, this plant can be brought 
into flower after etherization in from three to six weeks, 



336 



Plant Physiology 



whereas twice as much time would be required without 
etherization. Later in the season the lilac is not so 
readily forced, and there is no such marked contrast be- 
tween the treated and the control plants. Material 




Fig. 88. Lilies of the valley, etherized (A) and unetherized (B), then 
grown under similar conditions for the same length of time. 

capable of beginning growth immediately cannot be 
forced in this way. 

There is some contradictory evidence respecting the 
etherization of bulbs, but in general, the practice has been 
successfully employed with lilies of the valley, narcissus 
and daffodils, and certain lilies and tulips ; but the best 



Growth 



337 



results in the United States have been obtained with lilies 
of the valley. In many cases, the treated bulbs have been 
brought into perfect blossom two or three weeks earlier 
than normal. 

Howard determined that etherization will incite to more 
rapid bud activity a large number of common deciduous 
trees. He employed in one interesting series of experi- 
ments, shoots from 70 species of trees, including many 

Experiments with 70 Species of Trees and Shrubs 



Treatment 


Time in 
Days to 

begin 
Growth 


Time in 
Days to 

FULLY 

open Buds 


Per Cent 
which 

GREW 


Per Cent 
whose Buds 

unfolded 
completely 


Control 


21.5 


28.1 


58.5 


44.2 


Etherized 48 hours . . 


13.1 


20.3 


62.8 


50.0 


Etherized 48 + 48 hours 1 . 


12.7 


18.3 


47.1 


35.7 


Dried 1 day 


20.7 


26.2 


52.8 


45.7 


Dried 5 days 


13.8 


18.7 


35.7 


32.8 


Frozen 8 days .... 


18.3 


23.8 


47.1 


22.8 


Frozen 14 days .... 


16.4 


23.8 


14.2 


11.4 


Darkness 8 days . . . 


22.8 


29.1 


55.4 


34.2 


Darkness 14 days . . . 


23.9 


29.2 


58.5 


32.8 


Frozen 8 days 1 
Etherized 48 hours J 


11.5 


15.5 


31.4 


15.7 


Frozen 8 days } 
Etherized 72 hours 1 


9.9 


16.4 


30.0 


21.4 


Frozen 8 days ) 
Darkness 5 days ) 


17.5 


25.0 


21.1 


20.0 


Darkness 8 days } 
Etherized 48 hours J 


20.5 


26.5 


65.7 


38.5 


Darkness 8 days } ■ 
Etherized 72 hours J 


18.2 


25.8 


48.5 


35.7 



1 Interrupted exposure. 



338 Plant Physiology 

species which are known to be difficult to force into 
activity. In this test there were employed several species 
of Acer, Alnus, Azalea, Castanea, Cornus, Crataegus, 
Fraxinus, Populus, Quercus, Tilia, and Ulmus, besides 
many genera represented by a single species. Shoots from 
these plants were brought into the greenhouse at Halle, 
Germany, from December 8 to 23. The preceding table 
indicates the result of the etherization processes, and also 
compares this method of forcing with others involving 
change of conditions. 

198. The effect of etherization. — There are as yet no 
such definite indications as will permit a competent expla- 
nation of the effects of etherization upon the plant. By 
some the treatment is assumed to cause a stimulation, 
and no further suggestion is made. The view is also ad- 
vanced that there is an indirect effect upon the stored 
food. Again it is assumed that there is a loss of water 
from the cells, equivalent to a considerable time factor in 
the general maturity process. There is apparently no 
experimental work to confirm this view, and no ordinary 
method of desiccation is so promptly effective. It is 
more probable that the permeability of the protoplasm is 
directly influenced. 

199. Forcing by immersion in warm water. — In order 
to start into more rapid and certain growth dormant stock 
for transplanting, it has long been the custom with some 
gardeners to immerse the roots in warm water. Recently 
Molisch has reported many interesting experiments based 
upon a practice of forcing by means of warm water. 
The method is applicable to most plants commonly ether- 
ized, such as lilac, mimosa, Forsythia, and bulbs. He has 



Growth 



339 



also found it possible to force in a similar way several 
hardy shrubs. 

The method consists, in general, in immersing the plant 




Shoot of lilac : branches to the right forced by the hot-water 
method of Molisch ; branches to the left, control. 



340 Plant Physiology 

or branch in water at a temperature of from 30 to 35° C. 
for a period of from 9 to 12 hours. When potted plants 
are employed, it is preferable to invert the pot and im- 
merse the stem portion only, since the roots are generally 
more sensitive to injury. This method has certain prac- 
tical advantages over etherization, and if as generally 
successful, it will doubtless become important. The 
changes brought about by this treatment have not been 
determined. 

200. Transplanting after wilting. — Practical truck 
growers are often met who are in the habit of wilting cer- 
tain seedlings before transplanting, claiming that plants 
thus wilted recover promptly and grow off more vigor- 
ously than others not so treated. Experiment seems to 
confirm the practice for the tomato, and it may be sug- 
gested, provisionally, that the effect is indirect. A rather 
rough removal of tomato seedling from the seed bed 
results in some injury to the rootlets and root-hairs. If 
wilted, these roots do not recover upon transplantation, 
and vigorous new roots are promptly developed under 
suitable conditions. 

On the other hand, it appears that in the case of those 
seedlings placed without wilting under more favorable 
conditions for growth, the injured roots may recover slowly, 
and generally new roots are not so promptly developed. 
It may, perhaps, be inferred that any plants which do 
not readily develop new roots, such as the lettuce, corn, 
etc., would be greatly injured by the wilting process. It 
seems certain that transplanting with so great a ball of 
earth as not to injure the rootlets would be preferable in 
all cases, except where the roots are so much entangled as 



Growth 341 

to require being set free. It is not at all evident why 
wilting may be favorable to many cuttings, unless, perhaps, 
there is a tendency to permit too many leaves to remain 
on the cuttings, the vigorous activity of which is then 
permanently checked or inhibited by the wilting. 

201. Growth movements. — Growth movements of 
the varied sorts known may be referred to two types. 
These are (1) autonomic, or those resulting from internal 
and generally unknown conditions, and (2) paratonic, 
resulting as a response to external conditions or stimuli. 
Such movements are discussed in the special chapter on 
growth movements, also in those chapters dealing with the 
relations of plants to single environmental factors. It is 
sufficient here to note that there are various types of 
growth movement. 

LABORATORY WORK 

Elongation of root and shoot. — Determine the growing region 
of roots of the horse bean, bean, or field pea. Use germinating 
seed in which the radicle has developed to the extent of from 1 to 
li inches. With a fine thread dipped in India ink mark off parallel 
lines at equidistant intervals, of from 1 to 2 mm., placing the 
first mark in one or two cases as near the root-tip as possible and 
in other specimens at a full interval from the tip. Make daily 
observations and measurements and give a table or plot curve 
of the results. In order that the marked seedlings may be kept 
under suitable conditions, place each in the bell of a thistle tube 
(containing a little moist moss) with the root extending into the 
tube, the lower end of the latter resting in water. Favorable 
conditions may also be secured by pinning the seed, with the 
roots projecting vertically, to the bottom of a large cork to which 
has also been fastened moist filter or blotting paper. The cork 
is then fitted into a tumbler containing some water. 



342 



Plant Physiology 



Mark off also convenient (about 2 mm.) intervals on several 
of the younger internodes of plants of Phaseolus growing in soil 
and determine the region of elongation of the stem. Determine 
also the total growth in successive internodes of a mature plant 
and develop a graph of the results. 

Remove carefully the leaves from a node or two of half -grown 
oats or rye, mark off parallel lines on the internodes both near 
the basal and the upper parts, and describe the elongation phe- 
nomena, preserving the plants, if possible, under moist con- 




Fig. 90. Force of growth in the ostrich fern : loaves breaking through 
concrete pavement. [After Stone.] 



ditions. Study and prepare a curve of growth for the scape of 
dandelion, using plants growing either in the field or under green- 
house conditions. 

Extension of leaves. — Study the rate of development of broad 
leaves such as those of grape, squash, or bean, measuring on 
successive days or periods both length and breadth. 

Secure branches of one or more trees in winter condition, such 
as lilac, beech, poplar, and apple. Determine the average 
number of nodes produced by a season's growth and compare this 
with the number of nodes or leaves found by the dissection of 
half a dozen buds. 

Growth in tissues. — Dissect out the growing point of Elodea 
or Hippuris, mount in water under a cover glass, and examine. 



Growth 343 

Describe the formation of leaves. From prepared slides study 
and draw the growing tip in longitudinal section. 

From hand sections or from prepared slides study the secondary 
thickening in the stem of sunflower, castor-bean, or other plant 
of similar texture. Make some sections near the apex of the 
growing shoot and some farther distant in order to follow the 
development of inter-fascicular cambium and secondary bundles. 
From prepared slides measure the extent of variation in the 
growth of the seasonal rings. 

Adventitious organs. — (Roots.) Follow the development of 
adventitious roots upon cuttings of tomato, geranium, or grape. 
In the case of tomatoes in fairly dry soil this is also conveniently 
studied by binding to the stem at a node a ball of moist moss. 
Germinate sunflower seeds and as soon as the radicle has emerged 
about \ inch cut off the latter about \ inch from the coty- 
ledons, place the cotyledonary portions on moss in a moist cham- 
ber, and note the method of origin of the roots. 

(Buds.) Grow seedlings of flax in a saucer of sand or soil 
until the hypocotyls have about reached full growth. Then cut 
off the upper portion of the plant about l inch below the coty- 
ledons and discard the leafy portion. Cover the rooted hypo- 
cotyls with a bell glass or tumbler to prevent drying out and 
follow and describe the development of buds. Study the fleshy 
root of sweet potato to ascertain if preformed buds are present. 
Halve the root, place it upon moist sand under a bell glass, and 
observe the development of shoots. Examine the leaves of 
Bryophyllum calycinum for the presence of buds in the indenta- 
tions of the margin. If no buds are found, place the leaves on 
moist sand and observe occasionally. Follow likewise the 
development of buds from a leaf of Begonia Rex, placing the leaf 
upon moist sand with the petiole or a small part of the leaf slightly 
covered. Sever a few of the larger veins, and protect the leaf 
from drying out. 

Hot water forcing. — During early winter or midwinter im- 
merse for from 6 to 12 hours twigs of lilac (generally good) and 
apple in a water bath controlled at a temperature of 35° C. Re- 
serve some twigs untreated, or immerse them in water at 20° for 



344 



Plant Physiology 



control. Plan both sets under conditions favorable for growth, 
and compare the time periods required for development. It 
should be remembered that positive results are obtained by such 



IS 




HI 



Fig. 91. A convenient etherization chamber, sectional view, showing 
also carrier (c), groove for melted paraffin (g), and method of intro- 
ducing ether. 

forcing only when the plants treated are not in condition normally 
to show immediate growth. Potted plants may be inverted and 
immersed to the edge of the pot. 

Etherization. — In a tight zinc box such as shown in Fig. 91, 



Growth 345 

or in a chamber improvised from vessels at hand, etherize shoots 
or small plants of lilac and resting bulbs of lily-of-the-valley. 
Use about 40 grams of ether per 100 liters of space, and leave the 
plants in the chamber about 24 hours. Then place the plants 
with some untreated specimens under conditions favorable for 
growth, and compare the results. Write a short report stating 
your opinion of the extent to which such forcing may find 
practical application. 



References 

Askenasy, E. Ueber die jahrliche Periode der Knospen. Bot. 

Zeit. 35 : 792-815 et seq., pi. 4, 1877. 
Bailey, L. EL Lessons with Plants. (Cf. pp. 1-69.) 

The Pruning-Book. 537 pp., 33 figs., 1898. 

Busse, W. W. Beitrage zur Kenntniss der Morphologie und 

Jahresperiode der Weisstanne (Abiesalb a Mill). Flora. 

77: 113-175, pi. 3, 1893. 
Goebel, K. Einleitung in die Experimentelle Morphologie der 

Pflanzen. 260 pp., 135 figs., 1908. 
Gruss, J. Beitrage zur Biologie der Knospen. Jahrb. f. wiss. 

Bot. 23 : 636-703, 4 pis., 1892. 
Hartig, R. Das Holz der deutschen Nadelwaldbaume. 147 

pp., 6 figs., 1885. 
Haberlandt, F. Landwirthschaftliche Pflanzenbau. II Die 

Pflanze und ihr Wachsthum, pp. 139-296, 1879. 
Howard, W. L. Untersuchungen ueber die Winter-ruheperiode 

der Pflanzen. Inaugural-Dissertation, Halle, 111 pp. 
■ An Experimental Study of the Rest Period in Plants. 

Missouri Agl. Exp. Sta. Research Bui. 1 : 105 pp., 1910. 
Johannsen, W. Das Aether- Verfahren beim Fruhtreiben. 65 

pp., 1906. [Fischer.] 
Koopman, K. Grundlehren des Obstbaumschnittes. Landw. 

Jahrb. 25:497-618, pis. 5-28. 
Molisch, H. Das Warmbad als Mittel zum Treiben der 

Pflanzen. 38 pp., 1909. [Fischer.] 



346 Plant Physiology 

Moore, Emmeline. The Study of Winter Buds (etc.). Bui. 

Torrey Bot. Club. 37 : ] 17-145, pis. 9-11, 1909. 
Waugh, F. A. The Graft Union. Mass. Agl. Exp. Sta., Tech. 

Bui. 2 : 16 pp., 10 figs., 1904. (Compare also, Report 

21 : 174-192.) 

Texts. Barnes, Deimer, Garwng, Jost, Pfeffer, Stevens, Stras- 
burgcr. 



CHAPTER XIV 
REPRODUCTION 

The production of new individuals by any method 
whatsoever is reproduction in the broader sense. Physio- 
logically it is a complex and peculiarly interesting pro- 
cess. In the higher plants, — angiosperms and g} r mno- 
sperms, — we are concerned with reproduction by seeds 
and reproduction by vegetative parts. 

Seeds are embryonic plants with a certain food-supply 
and protective coverings, while vegetative parts may be 
shoots or any portion of the old individual which, when 
placed under favorable conditions, will develop shoot and 
root. The one type is usually sexual ; the other is invari- 
ably nonsexual. 

Vegetative reproduction generally implies (1) the ad- 
ventitious development of roots, as in cuttings, bulbs, 
the potato, etc., where buds are preformed ; or (2) of both 
root and bud, as in the sweet-potato, Dahlia, etc. Repro- 
duction by seeds involves commonly a variety of phe- 
nomena including the differentiation of new structures, 
the fusion of cells (gametes), and the origination of a new 
individual from a fertilized egg-cell. 

202. The seed habit and vegetative reproduction. — 
Reflection upon the general conditions prevailing among 
cultivated and wild plants leads to the conclusion that the 
347 



348 




Fig. 92. Apricot blossoms : 
growth from stored food. 



Plant Physiology 



production of seed is for most 
plants of paramount importance. 
Vegetative methods of repro- 
duction may also occur in plants 
possessed of the power of abun- 
dant seed production; and, in- 
deed, under favorable circum- 
stances the former may propagate 
individuals more rapidly. Sup- 
plementary vegetative methods 
of reproduction are therefore 
common. Wild onions and lilies 
may have their " sets " and 
bulbs. Numerous plants develop 
offshoots, root shoots, and natural 
layers, and so perpetuate them- 
selves in a variety of ways. A 
few plants both wild and cul- 
tivated, such as forms of the water 
weed and the yam, have entirely 
or practically lost the power of 
seed-making. In general, how- 
ever, the seed is the basis of plant 
production, although vegetative 
reproduction has been employed 
far beyond its natural course, and 
this in order to perpetuate a type, 
to multiply individuals quickly, 
and to grow plants under climatic 
conditions rendering seeding un- 
profitable or impossible. 



Reproduction 



349 



203. The flower : essential structures. — Richness of 
color or striking form and fragrance in flowers may serve 
useful ends leading toward reproduction. 
Moreover, in ornamental plants these 
qualities often represent the crop value of 
the plant. Beneath an apple tree in 
spring the ground may be white, strewn 
with discarded petals, representing much 
energy of growth, that was, nevertheless, 
serviceable. In seed production, however, 
it is stamens and pistils which are directly 
important, and the inconspicuous, unob- 
trusive, or unattractive flowers of spinach, 
lettuce, and corn are as effective as the 
beautiful or gaudy structures of the 
orchid and hollyhock. 

204. Pistil and stamen. — The pistil 
is commonly composed of one or more 
carpels. Whether consisting of one or of 
several carpels, it embraces in common 
types (1) the ovule-sac, generally a 
membranous or fleshy structure, contain- 
ing at the time of flowering the relatively 
small, seed-like ovules, or megasporangia ; 
(2) a more or less well defined style, upon 
the terminal portion or surface of which is 
differentiated (3) the stigma. 

The stamens consist in general of a stalk part or fila- 
ment, supporting the anther, which latter contains the 
anther sacs, or microsporangia, with their pollen-grains. 
Stamens and pistil may be present in the same flower, 




Fig. 93. Flower 
of barley. 



350 



Plant Physiology 



known as a perfect flower, of which the apple, cotton, 
wheat, etc., are examples. These structures maj- oc- 
cur in different flowers, termed staminate and pistillate 
flowers, upon the same plant, that is, monoecious (one 
household) plants, of which the corn and squash are 
examples ; but they may occur upon different individuals, 
that is, staminate and pistillate, or dioecious (two house- 
holds) plants, of which latter type the hemp, certain 



* fi\t5S 



iLJl? &** 



<'' : -j- 



■^Ifr 




Fig. 94. Carpels and stigmas (.4) of orchard grass ; also enlarged view 
of stigmatic cells and pollen germination. 

mulberries, and the date-palm are examples. In any case, 
approximately at the time the flowers are open, or mature, 
the anthers of healthy stamens may set free considerable 
pollen. At about the same time the stigma or stigmatic 
surface of the carpel is receptive; that is, generally, in a 
condition to catch or affix pollen-grains, and to afford 
special conditions for their germination. 

205. Pollination and pollen-tube penetration. — Pol- 
lination is a mechanical process. As it naturally occurs, it 



Reproduction 



351 



is the mere dusting of the stigmatic surfaces with pollen. 
This pollen may be derived from the anthers of the same 
blossom, from different flowers, individuals, or species. 




Fig. 95. Flowers of date-palm : staminate cluster (A) and blossoms (a) ; 
pistillate cluster (B) and blossoms (6). [After Swingle.] 



If such pollen-grains do not germinate, or if no germ- tubes 
penetrate the stigma, they may awaken no more response 
than so much dust. If we may acknowledge faith in the 
current belief regarding pollen as a factor in " hay fever/' 



352 



Plant Physiology 



then the pollen grains of the ragweed, of timothy, and of 
some other plants are among those forms which may 




Fig. 96. Staminate and pistillate blossoms of Begonia. 

as dust, or perhaps in some chemical manner, severely 
irritate the mucous membranes of man. However, the 



Reproduction 



353 



dusting of flowers with pollen would not be dignified with a 
name unless it led toward fertilization and reproduction. 

The stigmatic surfaces of the receptive flowers are 
generally moist, and often provided with a perceptible 




Fig. 97. Pollen grains and pollen germination; corn (a, b), apple (c), 
sweet-pea (d), and Althaea (/). 

secretion. Upon this surface the pollen may germinate, — 
almost any pollen may germinate ; yet it usually happens, 
from a variety of circumstances, that the pollen most 
abundant upon any stigma is the pollen of the same 
species. This is really what is generally implied by effec- 
tive pollination. Strasburger, however, has made the 
interesting discovery that upon any particular stigma pollen 
of a plant in an entirely different family may not only 
germinate, but may even penetrate the style to some extent. 

2a 



354 



Plant Physiology 



Normal pollination takes place through the agency of 
wind and insects, for the most part ; and it may be inter- 
fered with by rain or other climatic conditions not resulting 
in the death of the flower. Such conditions may close the 
flowers, preventing the transfer of pollen ; or beating rains 




Fig. 98. Pistillate blossom of squash, showing large stigmatic surfaces. 

may wash the pollen from the stigma. In consequence, 
orchards ma} 7 fail to be productive through the effect of 
climatic conditions upon pollination. 

206. Fertilization. — Fertilization is the union and fu- 
sion of two single gametic cells, or nuclei, which have 
previously been differentiated by a special course of de- 
velopment. In flowering plants the one gamete is derived 



Reproduction 355 

from the pollen-grain, the other from the embryo-sac in 
the ovule. In the fusion of these nuclei, usually derived 
from different organisms or flowers, the characters of two 
individuals are fused. Two lines of ancestry are brought 
together in one cell, the fertilized egg, or zygote, which 
will develop into the embryo of the seed. It is important 
to bear in mind some further details regarding the fertili- 
zation process. 

The pollen, as has been noted, is a distinct phase of the 
plant. It represents upon germination the complete 
male gametophyte, whose reproductive function is the 
production of a gamete. In most cases the pollen-grain 
consists of merely two cells, — a smaller cell practically within 
a larger. The larger produces through germination a vege- 
tative tube, the germ tube, which (in angiosperms) grows 
through a differentiated portion of the style, or stylar canal ; 
thence it penetrates the ovule, commonly through the 
micropyle, until it comes in contact with the egg-apparatus, 
and ultimately with the egg-cell. The smaller cell of the 
pollen-grain is largely nucleus. The latter divides by the 
time the pollen-tube breaks or ruptures, and one of these 
two gametic nuclei fuses with the nucleus of the egg-cell, 
the other gamete; thus fertilization is effected. The 
fertilized egg — this single cell, or zygote — is the be- 
ginning of the new individual that is developed within 
the protecting coats of the ovule (now the young seed), in 
turn inclosed by the ovule-sac, and often by other parts 
of the flower which may assist in the development of the 
fruit. 

207. Universality of fertilization. — It is remarkable 
how universal is this phenomenon of fertilization. It 



356 



Plant Physiology 



occurs throughout nearly all the phyla and classes of 
organisms, and is of unquestioned importance. It is not 
possible to consider the many interesting opinions regard- 




Fig. 99. Pistil of squash : cross-section of style (A), with region of pollen- 
tube penetration (a) ; portion of preceding, enlarged (B) ; longitud- 
inal section (C) corresponding to the preceding ; cross-section of ovule 
case (D) with region of pollen-tube penetration (a). 



Reproduction 



357 



ing the results and benefits of the process ; but emphasis 
should be laid upon the union of characters — amphimixis 
■ — thus effected. Reference is made 
later to the segregation of characters 
which is conceded to take place. 

Every ovule requires a pollen-grain 
and a pollen-tube. In fact, for fer- 
tilization of all the ovules in any 
plant, many more pollen-grains ger- 
minate and penetrate the style, since 
two or more tubes may be directed 
toward the same ovule. The water- 
melon may develop more than five 
hundred seed, so that more than a 
thousand pollen grains should fall upon 
a single stigma to insure the maturity 
of all the seed. 

All the ovules may be fertilized, 
yet this does not guarantee fruit devel- 
opment of all. Frequently the plant 
would be unable to support the weight, 
or growth demands, resulting from the 
development of every fruit. 

Unfertilized blossoms are usually 
the first to fall, but familiar examples 
are evident on every hand of wild and 
cultivated plants which shed many 
fertile blossoms. Correlative growth 
influences, which are little understood, 
or an unfavorable environment, may take heavy toll as 
the young fruit develops ; so that at maturity only a small 




Fig. 100. Carpel of a 
legume, diagram- 
matic section at the 
time of fertilization. 



358 



Plant Physiology 



percentage of the flowers may have completed their 
functions. According to Waite, apples in good season set 

no more than 5 per 
cent of fruit, and 
of this small per- 
centage much is 
lost during fruit 
development. If 
we put this into 
figures, we find 
that 1000 apple 
blossoms may 
yield about 50 
young fruits, of 
which only some 
few reach matu- 
rity. In other 
cases, practically 
every ovule may 
mature, if fertili- 
showy, but often za tion is effected. 
This is particu- 
larly true of plants producing fewer flowers or floral axes, 
as the corn or strawberry. 

208. Cross-fertilization and self-fertilization. — These 
terms are used more or less loosely. Cross-fertilization 
generally indicates a fusion of gametes derived from differ- 
ent individuals, resulting, therefore, from pollination of a 
stigma with pollen derived from a different plant. To be 
consistent, self-fertilization would then indicate that both 
gametes are derived from the same individual. 




Fig. 101. 



Blossoms of cotton ; 
self-pollinated. 



Reproduction 359 

As a matter of fact, there are several grades of self- 
fertilization ; thus fertilization as a result of pollinating 
the stigma with pollen from the same blossom, from a 
different blossom upon the same plant, or from another 
plant derived by bud or scion propagation from the same 
" parent " stock. In much the same way cross-fertiliza- 
tion is a broad term, applying when the gametes are de- 
rived from any two individuals (grown from seed) within 
the species; that is, whether the crossing is between indi- 
viduals from pure lines, from merely mixed seeds, or from 
distinctly different strains or races. 

When reference is made merely to the dusting with 
pollen, the terms self and cross pollination should be 
employed, but many authors writing popularly fail to 
make these distinctions. 

209. Cross-fertilization apparently the rule. — Cross- 
fertilization is a phenomenon of common occurrence with a 
considerable number of ecologically well-established native 
species of plants, and vigorous cultivated varieties as 
well. It is evidently effective, but it is by no means uni- 
versal among seed-plants. If we accept the analysis 
which has thus far been made, it is, however, far the more 
common method among flowering plants. Cross-fertiliza- 
tion is, of course, dependent upon cross-pollination, and 
both are commonly associated with the remarkable 
developments in form, color, and other characteristics of 
numerous familiar flowers to which popular attention has 
been so much attracted. Nevertheless, it should not be 
understood that these striking peculiarities of floral 
structures are in strict correlation with cross-pollination. 
Dates, mulberries, hops, and hemp are invariably cross- 



360 Plant Physiology 

pollinated and cross-fertilized when fruit is produced, 
because in these species stamens and pistils are on distinct 
individuals. In corn there is opportunity for self-fer- 




Fig. 102. An car from an isolated stalk of corn ; infertility from lack of 
cross-pollination. 

tilization, but crossing is the rule. In fact, isolated stalks 
of corn seldom set more than scattering grains (Fig. 102). 
Darwin's observation respecting the necessity of cross- 
pollination in red clover has become a familiar instance 
among perfect flowers. He demonstrated that the heads 
of this species protected from bees and other insects set 
no seed. This may not be due, in the case of clover, to 
the ineffectiveness of the pollen of each particular blossom 



Reproduction . 361 

upon the stigma of the same flower, but rather to struc- 
tural difficulties preventing pollination; that is to say, 
it appears that the flowers may be self-f ertile ; nevertheless, 
the effect is that the best seed production requires insect 
visits. In consequence, to produce clover seed economi- 
cally, this crop should be permitted to flower only in the 
season when the bumblebees are abundant and active. 
The first crop is too early, so that it is commonly cut for 
hay, and the second crop is permitted to develop seed. 

210. Darwin's conclusions. — A study of the remark- 
able morphological devices in many flowers pollinated by 
insects suggested to Darwin the importance of determin- 
ing, with respect to the offspring, the comparative physio- 
logical effects of cross and self fertilization. He made 
fertility and constitutional vigor of the offspring his field 
of investigation. As a result of his extensive experiments 
with some familiar plants throughout a period of years, 
Darwin concluded that " cross-fertilization is generally 
beneficial, and self-fertilization injurious." In drawing 
these conclusions, he made careful comparisons of the 
offspring with respect to height, weight, constitutional 
vigor, and fertility. 

Darwin recognized some general exceptions, in which, 
for instance, self-fertilization was often more effective for 
a generation or two (as in tobacco and Petunia) than 
crossing with relatively closely related individuals. 
Nevertheless, in the case of tobacco a cross with wholly 
fresh stock was invariably more effective than self-fer- 
tilization. The most notable exception to his general 
statement, quoted above, occurred in a vigorous individual 
of the morning glory (Ipomcea purpurea), Hero, whose 



362 Plant Physiology 

descendants " varied from the common type, not only in 
acquiring great power of growth and increased fertility 
when subjected to self-fertilization, but in not profiting 
from a cross with a distinct stock ; and this latter fact, if 
trustworthy, is a unique case, as far as I have observed 
in all my experiments." 

211. The need of further work. — The question of cross 
and self fertilization (cross-breeding and in-breeding) is 
now receiving renewed attention. The indications are 
that with many plants in-breeding is for plant production 
far less dangerous than has been supposed. It seems to 
be conspicuously dangerous in the case of corn, some 
reasons for which will be subsequently considered. Sha- 
mel states, " In the breeding of tobacco it is well known 
that cross-pollination within the limits of a single strain 
produces inferior offspring, and only self-fertilization gives 
offspring of the highest degree of vigor, though hybrids 
between distinct strains of tobacco often display a vigor 
superior to that of either parental strain. Examples 
could be continued indefinitely, but even one instance, in 
which long-continued in-breeding results in no injurious 
effects, would be sufficient to discredit the old hypothesis.' ' 

It is evident that in breeding studies each crop must be 
examined with respect to this important point. It is to 
be expected that much new evidence on the general prob- 
lems of self and cross fertilization will be available, for 
the more certain methods in recent years with pure-line 
ancestry, the conception of unit characters, the develop- 
ment of biometry — all make possible far more definite 
experimental conditions. 

212. Experiments with self-sterility in pear. — Waite 



Reproduction 



363 



established an important principle in fruit-growing when 
he showed that many varieties of the pear are self-sterile. 
In the case of some varieties the capacity to set fruit when 
self-pollinated is wholly lacking ; in other cases when the 
varieties are limited to their own pollen, normal fruits 
may be developed, yet even then fruit production is con- 




Fig. 103. Difference in size of pears self (a) and cross pollinated (6). 
[After Waite.] 

sidered to be less certain and the size often slightly re- 
duced. These results, and many others since reported by 
various observers, are particularly interesting, since self- 
pollination has reference not merely to the use of pollen 
from the same plant, but also from other individuals within 
the variety. We are dealing, in this case, with clonal 
varieties, such varieties being maintained by budding and 
grafting. Different individuals are looked upon, therefore, 
as very closely related. In most instances all are de- 



364 Plant Physiology 

scended from some one ancestral or stock individual, and 
there have been, so to speak, nothing but generations of 
this original individual. Pollination between the different 
trees in a block of Bartlett pears would, therefore, be con- 
sidered self-pollination. 

According to Waite, some of the common varieties of 
pear generally self-sterile are the following : Angou, Bart- 
lett, Clairgeau, and many others, 22 in all. The varieties 
showing a capacity for self-fertilization were 14 in number; 
among these being the Flemish Beauty, KeifYer, Le Conte, 
and Seckel. Climatic conditions have been shown to be 
of some importance with respect to the general problem 
of self-sterility (cf. Fletcher). 

213. Self-sterility in other orchard trees. — The apple, 
plum, peach, and other fruits are similarly more or less 
self-sterile. The investigation of self-sterility, which was 
given a special impetus tnrough the work of Waite, has 
resulted in a modification in orchard-practices of im- 
mense economic value. It is certain that the grower 
needs to consider an adequate distribution of varieties 
so that pollination with effective pollen may be secured to 
all. A particular variety may not be constantly self-sterile 
under diverse conditions, and it is clear that many prob- 
lems respecting pollination await careful investigation. 

214. Parthenogenesis. — There are exceptions to the 
rule that the egg cell must be fertilized in order to develop 
the embryo of the seed. The maturation of a cell occupy- 
ing the position of an egg with or without the usual segre- 
gation, or reduction, division and its development without 
fertilization constitute parthenogenesis. This phenome- 
non is reported to be characteristic of several forms of 



Reproduction 365 

dandelion; and there are similar instances among hawk- 
weeds, meadow rue, Alchemilla, and several other genera 
of flowering plants. In fact, if we examine the cases re- 
ported for all plants, both higher and lower, we find that 
parthenogenesis, or that which is here included under that 
term, is not uncommon. Development without fertili- 
zation is a well-established phenomenon in certain insects 
and other lower animals. There are also some extremely 
interesting cases of what has been termed artificial par- 
thenogenesis, reported by Loeb, Lefevre, and others. 
In the latter the artificial development of the egg-gamete, 
without fertilization, is induced by chemical or physical 
stimuli. When there is no tendency toward natural 
parthenogenesis, this artificial stimulation of development 
has never gone so far as to produce adult individuals, but 
larval stages have been successfully reared. 

215. Xenia in corn. — The appearance of the seed 
gives usually no indication of the pollen which was effective 
in fertilization. In corn, however, the observation was 
recorded nearly two centuries ago that certain colored 
sorts planted together " will mix and interchange their 
colors " ; that is, in the seed of the first year there will be 
mixed colors. Numerous carefully conducted experiments 
in recent years by De Vries, Correns, Webber, and others 
now clearly demonstrate the immediate effects of the pollen 
in corn, and a physiological explanation is at hand. 

215 a . Indications of xenia. — We may first note the re- 
sults of xenia. If pollen of the black Mexican or Cuzco 
varieties, forms possessing a bluish-black aleurone layer, 
are applied to the silks of white or yellow sorts, many of 
the seed resulting show the blue-black color of the pollen 




Fig. 104. Xenia in corn, showing the immediate effect, through double 
fertilization, of the pollen-producing parent. [Photograph from Bureau 
of Plant Industry.] 



Reproduction 367 

plant. Again, if the young ears of sweet corn are pol- 
linated with a pollen from Dent and Flint varieties, there 
result many seeds with smooth kernels and starchy endo- 
sperm (Fig. 104). These results may be secured with pure- 
line strains under control conditions, and the phenomenon 
is recognized as xenia. Characters such as those above 
noted alone exhibit true xenia; thus color or chemical 
content, qualities which reside in the endosperm, are of 
this nature, while qualities evident through the embryo 
belong to another category. The explanation of this en- 
dosperm phenomenon has now been found in the process 
of double-fertilization. 

Generally the nucleus and cytoplasm of the embryo-sac 
develop the endosperm, and only the pistil-bearing plant 
is concerned with the qualities of this material. In the 
lily, in corn, and in many other cases more recently made 
known, the nucleus of the embryo-sac may fuse with the 
second sperm nucleus from the pollen-tube, and thus the 
endosperm may acquire, as well as the embryo, qualities 
of the pollen-producing plant. It is evident that corn 
which possesses color by virtue of a pigment in the pericarp 
will not show this type of phenomenon, for the pericarp has 
no means of becoming immediately endowed with the 
qualities brought by the pollen. Cases of xenia, therefore, 
should not be confused with ordinary speckled ears. The 
latter may result in the second generation, or later, from 
crosses in which color is one of the characters of one or 
both parents, or from bud variation. Xenia is a question 
of physiological interest in plant production, but it has 
apparently very little practical bearing. When it exists, 
it is an infallible sign of hybridization, and in some cases 



368 Plant Physiology 

it may serve as a valuable control suggestion in hybridi- 
zation work. 

216. False xenia. — Certain beans and peas show ex- 
ternally in the color of the embryo the immediate effects 
of the pollen. The seed-coats are more or less colorless, 
and the characteristics of the embryo are, therefore, ap- 
parent. Physiologically this is in no way comparable to 
the previous phenomenon, and if the term " xenia " is 
used in this connection, it should be expressed false xenia. 

217. Other secondary effects of pollination. — Under 
xenia we have considered only those instances of the sub- 
sidiary effects of the pollen which may be attributed 
directly to fertilization, or double fertilization, and 
manifest through effects produced in a readily explained 
manner upon the embryo and endosperm. A certain 
special stimulating action of pollination upon structures 
outside of the ovule was long ago suggested by Focke and 
others. The problem may, for the moment, be restricted 
to cases in which there is fertilization. The question may 
then be formulated as follows : In the normal production 
of fertile seeds, is there any evidence that pollen from 
different varieties or species will influence the form, color, 
or quality of the fruit ? 

Beyond all question, the form of the fruit, and even the 
quality of the fruit, will be affected when only a few ovules 
are fertilized; for the reason that there will be incomplete 
development of the fruit as a whole in the great majority 
of plants, notably in many varieties of the tomato and 
apple. Opinions differ regarding the important effect of 
pollen from different varieties on the form or color of fruit 
w r hen fertilization is complete. In various horticultural 



Reproduction 369 

reports it has been stated that very often a definite effect 
is due to the source of the pollen. An analysis of the data 
seemed to indicate that the observations which have been 
made neither positively confirm nor deny a direct and 
specific stimulation of the pollen upon the fruit-production. 
In the paper previously referred to, Waite draws this 
conclusion: " There seemed to be, however, constant 
differences between the Bartlett fruits crossed with dif- 
ferent kinds of pollen. If these distinctions can be con- 
firmed by future experiments, a question of considerable 
importance will be settled." Lewis and Vincent seem to 
concur in the belief that there is an immediate effect of 
the pollen, and they cite the deep red color in Spitzenberg 
apples pollinated with Arkansas Black as compared with 
the lighter red obtained when Jonathan is the pollenizer. 

218. Parthenocarpic development. — It is considered 
to be a general rule that lack of fertilization is followed by 
more or less prompt shedding of the infertile blossoms. 
There are, however, important exceptions to this course of 
development. Seedless fruits of garden and orchard crops 
are known. Seedlessness, or imperfect seed development, 
is very properly associated with the failure of fertilization, 
although it may happen that some ovules fail to develop 
after fertilization. On the whole, it seems to be clearly 
demonstrated that in some cases the ovary and attached 
parts, technically the fruit, may develop as a purely vege- 
tative organ, varying more or less, of course, in size from 
its normal form when fertilization has taken place. 

Among vegetables the cases of parthenocarpic develop- 
ment best known are those of the English forcing cucumber 
and certain varieties of the eggplant. Gardeners fre- 



370 Plant Physiology 

quently prevent the pollination of the forcing cucumber, 
especially when it is to be used for fancy market purposes. 
Certain varieties normally show the development of seeds 
toward the apical end of the fruit only, and this part is 
then more or less abnormal in size, so that the general form 
of the plant is injured. It would appear that in the case 
of the eggplant a relatively small number of blossoms will, 
under ordinary circumstances, develop fruit without seeds, 
and Munson succeeded in obtaining fruits of normal size 
and form. Many other observations might be cited of the 
occasional appearance of seedless vegetables, but our chief 
interest should be directed towards certain observations 
upon fruits. 

219. Parthenocarpic formation in pomaceous fruits. — 
Waite ascertained that in certain cases self-fertilized 
pears " are deficient in seeds, usually having only abortive 
seeds, while the crosses are well supplied with sound 
seeds." In fact, there were two significant exceptions to 
the rule requiring pollination and fertilization for fruit 
development. Upon the Le Conte and Heathcoate 
varieties a few fruits were set without pollen. Still, in 
those two instances, there was some doubt as to the com- 
plete exclusion of pollination. A few isolated instances 
of the occurrence of pears, apples, and other pomaceous 
fruits without seed have been recorded, but the possibil- 
ity of perfect fruit development in certain varieties, or 
the development of varieties which may not require fertili- 
zation, has only recently been carefully investigated. 
Ewert for one has made this matter a subject of experi- 
mental study, and it would appear that when pollination 
is prevented, perfect fruits may result in the Cellini and 



Reproduction 



371 



Charlamowski apples, and in six varieties of pears, of 
which the most important is the Clairgeau. In the best 
of the cases reported by him the fruits suffered neither 
diminution in size nor change of quality, although parthen- 
ocarpic development is commonly accompanied by de- 
creased size. Sections of well-fertilized and seedless 
fruits are shown in Figure 105. 




Fig. 105. Section of fertile (seed-bearing) and seedless (parthenocarpic) 
apples. 

219 a . Seedlessness in the orange, grape, and banana. 

— Well-established varieties of citrus fruits, commercially 
important, normally produce no seed. The California 
navel oranges are of this type. In this instance pollina- 
tion cannot lead to fertilization, since, according to the 
reports available, the stigmatic surface apparently fails 
to reach full development, or to become normally exposed, 
and thus germination and the entrance of the pollen tubes 
is precluded. Further observations upon this point are 
needed. 

Historically of more interest are certain cases of par- 



372 



Plant Physiology 




thenocarpy among grapes. The small seedless black or 
currant grapes of Greece and the eastern Mediterranean 
region furnish the dried cur- 
rants of commerce. Other in- 
stances are to be found in the 
famous Sultana and a few other 
raisins now propagated in Cali- 
fornia as well as in the Medi- 
terranean region. The percent- 
age of seedlessness in one of 

Fig. 106. Seedless navel (par- 

thenocarpic) and common the table grapes, Black Eagle, 

(seed-bearing) orange. } s likewise considerable. The 

cause of seedlessness, or lack of fertilization, in these 
cases does not seem to have received scientific attention. 

Many varieties of the banana fail to set seed, although 
it would seem that effective pollen is produced. 

220. Nonsexual reproduction. — Multiplication by 
vegetative parts is notably common among plants, wild 
and cultivated. A single individual may in various ways 
give rise to a colony or complete plantation of its own kind 
without the production of seed. These may remain in 
organic connection for a period of time, or they may be 
promptly separated, one from another, by the death of 
connecting parts. Wheat and other cereals and grasses 
have the habit of stooling; that is, of multiplying by buds 
from the lower submerged nodes. Blue-grass and John- 
son-grass are among those plants which produce under- 
ground stems, while the decumbent Bermuda and quack- 
grasses are among those which regularly take root at the 
joints. White-clover spreads in a way analogous to the 
latter, and the strawberry develops runners which bud 



Reproduction 373 

and take root a few inches or more from the parent plant. 
Advantage may be taken of all such nonsexual methods 
in practical propagation. Moreover, in all such cases the 
vegetative method enables the producer to be sure of the 
variety or form, the propagation of which he is continuing, 
since there is then, at least, no chance of mixing by hy- 
bridization, or of change through segregation. As a rule, 
plants that reproduce in a vegetative manner occupy the 
land quickly. The method is, of course, of great service 
when the plants are useful, but it may be most trying when 
this habit is that possessed by a persistent weed. 

221. Thickened roots and tubers. — Irish potatoes and 
Madeira vines are types of plants propagated by tubers or 
thickened stems, produced generally only by underground 
buds. In the varieties of the potato commonly grown the 
seed-ball is now seldom seen, so that there are varieties 
the existence of which, in culture at least, is dependent 
upon vegetative reproduction. Sweet potatoes, yams, 
dahlias, and other familiar plants are propagated by 
thickened roots. Many of these forms have in cultivation, 
under ordinary conditions, lost the power of seed produc- 
tion. The propagation of some edible and many floricul- 
tural plants by bulbs and corms is so common among 
liliaceous genera that the production of bulbs now repre- 
sents a group of special industries. Holland is famous the 
world over for this type of work, but doubtless the condi- 
tions there afforded are practically duplicated in many 
other places. 

222. Cuttings. — A countless array of ornamental 
herbaceous forms and some small and bush fruits are 
regularly propagated by cuttings. In fact, carefully 



374 Plant Physiology 

handled, there are few herbaceous plants of indefinite 
growth which may not be successfully multiplied in this 
manner. In general, the growth relations are simple. 
The propagator employs a small portion of a branch con- 
taining one or more nodes. These nodes with active or 
dormant buds are capable of developing shoots. The 
essential response demanded of the plant is that under 
conditions favorable for growth it shall be able to develop 
adventitious roots. Such roots are by no means uncom- 
mon in nature, and they develop apparently more or less 
in correlation with the needs of the plant. The extent to 
which plant parts are able to develop such roots is re- 
markable. It has been ascertained that cotyledons and 
leaves are in no small number of cases able to develop 
these roots as efficiently as stems, and the leaves could, 
therefore, be employed in propagation if there were also 
the possibility of bud development. In fact, there are a 
few types which have the habit of producing buds, and 
such leaves are made use of in this manner. The leaves 
of several species of Begonia, notably the varieties of 
Begonia Rex, also certain forms of Bryophyllum and related 
plants, are thus employed, as already indicated. 

223. Precautions with cuttings. — It is evident that 
branches or shoots used as cuttings require, in general, 
careful treatment. Twigs or canes in a resting condition 
may require no special consideration, so that currants, 
grapes, and other fruits root readily under field or garden 
conditions. Throughout the South, sugar cane is gener- 
ally propagated by planting the whole stalks, and also by 
using the lower parts of stools, — the rattoons. Shoots 
in full leaf always require more attention, since the loss of 



Reproduction 375 

water (transpiration) may be excessive and death by dry- 
ing-out is then a chief cause of failure. In all cases the 
reduction of the transpiratory surface to a minimum is 
required, and it is essential that the conditions of the cut- 
ting bench shall be most favorable with respect to moisture, 
drainage, light, and temperature. The exposed cut sur- 
faces are also more subject to the attacks of hemi-parasitic 
or damping-off fungi ; therefore, the cutting bench re- 
quires the same careful attention as the seed-bed. In 
many cases a thorough knowledge of the growth habits of 
the plants and the best skill of the gardener will be required 
to determine the conditions needed. It may be neces- 
sary by special means to induce root development in the 
portion to be used for a cutting before the branch is sepa- 
rated from the stalk, as by attaching a pot with moist soil 
or moss. Special cases, however, are too numerous to 
receive consideration. 

224. Vegetative reproduction and running out. — Some 
observers have held that vegetative reproduction repeated 
through numerous generations results in deterioration; 
but many or all of the cases cited in substantiation of this 
view are, in the opinions of others, wholly invalid. Never- 
theless, in one sense varieties may " run out." Thus bud 
variation may be so great that in time the original form 
may be entirely lost; this is " varying out." 

It would seem that the yam (Dioscorea sativa) has been 
vegetatively propagated in China for two thousand years, 
and there is no evidence that it is decadent. The sweet- 
potato has apparently long lost the power of seed produc- 
tion, and we cannot assume that it has lost in vegetative 
vigor. The fig and the date have not been commonly 



376 Plant Physiology 

grown from seed for several centuries. The European 
grape-vine ( Vitis vinifera) has been grown more than 5000 
years, and vegetative propagation has been the rule. 
When native American vines were carried to Europe, 
diseases new to Europe were introduced, and European 
vines were so susceptible that enormous injury resulted. 
Many thought that this weakness with respect to disease 
was a result of the long vegetative culture ; but this seemed 
to be disproved by the fact that seedling sorts of that 
species of vine were equally susceptible. 

Resistance or susceptibility to any disease appears to be, 
in many cases, a character, or a complex of characters, and 
may follow known laws of heredity, as in the case of other 
characters subsequently discussed. Moreover, among the 
more familiar molds and other fungi there are some notably 
ubiquitous and vigorous forms which are not known to 
possess sexual stages. Among the species of living things 
generally, however, the frequency of gametic fusion, on 
the one hand, and the complete loss of this process among 
others constitutes a biological paradox. 

225. Relation of vegetation to fruiting. — Plants exhibit 
a remarkable diversity in the relations between vegetative 
development and fruiting. With respect to annual, 
biennial, and perennial habits this has been briefly con- 
sidered. Generally speaking, fruiting is the climax of a 
continuous or interrupted period of vegetative develop- 
ment. The American Agave grows many years vegeta- 
tively, and then through the formation of an enormous 
flower-stalk and abundant fruits the leafy parts are drawn 
upon to such extent that they are left exhausted and 
incapable of recovery. 



Reproduction 377 

Under exceptional conditions fruiting of the oak or 
apple may occur in the nursery stock. On the other 
hand annuals may be induced to grow for a period of 
years without flowering. With a careful selection of 
conditions, and by employing vegetative propagation when 
necessary, Klebs was able to induce continuous growth 
and fruiting in Parietaria officinalis. Uninterrupted 
growth, without flowering, was obtained with Fragaria 
lucida, Glechoma hederacea, Rumex acetosa, and other 
species, — plants which normally produce blossoms in 
summer. These facts suffice to suggest the complexity 
of the relations, and the importance of determining the 
releasing stimuli, with respect to vegetation and fruiting. 

LABORATORY WORK 

The flower. — Review or study the morphology of the flower, 
giving attention to monoecious and dioecious plants as well as to 
those with perfect flowers. 

Study more completely the floral mechanism in two or three 
representatives of some one order, such as the Liliaceae or Legu- 
minosse. [Consult Church or some other convenient text.] 

Anther and pollen. — Cut crosswise the large anthers of some 
plant, such as lily or tomato, press out the pollen-forming areas, 
and note the changes in the character of the contents of the anther 
(pollen) sacs, or microsporangia as maturity proceeds. 

Study and describe the pollen from plants in at least two 
different orders, mounting it both dry and in water. 

Set up germination experiments with pollen from several plants 
which produce tubes readily (within a few hours) in water or 
sugar solution. Germinating well in 3 per cent sugar solution 
there are among the numerous monocotyledons which might be 
used, several species of lily, also orchids, tulip, and Narcissus ; 
while cucumber, buttercups, willows, and Erica are among favor- 
able dicotyledons. 



378 



Plant Physiology 



The pollen grains may be sown in a drop of the sugar solution 
on the slide, which is then placed in a moist chamber. Prefer- 
ably, however, prepare hanging drop cultures (Fig. 107) made 
of glass rings cemented to the slide with wax, over each of which 
is inverted a cover-glass with a drop of the solution, in which the 
grains are sown. In the bottom of the cell is placed the same 
solution as employed in germination, and upon the upper rim 
of the ring is a thin layer of petrolatum in order to afford a closed 
chamber. 

Pollen of corn and some other grasses, also many sedges and 
rushes, germinate best in a moist atmosphere, and these may be 
sown on a dry cover-glass inverted over a cell containing water. 




Fig. 107. Hanging-drop cultures, used in the study of pollen germination. 

Pistil and fertilization. — Follow the changes in the stigmatic 
surfaces of several flowers as they open. Trace the canals or 
modified tissue through which the pollen tube penetrates. In 
the lily, squash, or cucumber, and many other plants the pollen 
tubes are readily seen in longitudinal section. 

If prepared slides are available, study the morphological evi- 
dences of fertilization. 

From the open buds of any plants convenient dissect out the 
stamens ("emasculate") before the pollen is matured, or the 
stigmatic surfaces exposed ; inclose the flowers in paper bags, or 
oiled paper, and after a week or more determine the effect upon 
ovule development and seed production in comparison with 
control plants. With a dioecious plant, such as Indian corn, 
merely protect from pollination the pistillate axis or ear. 

Fruit setting. — In the proper season make a careful count of 
the number of blossoms produced by such plants as the apple 
or peach, and later determine the percentage of fruit which may 
be set. 



Reproduction 379 

Xenia. — ■ Examine ears of corn from plots in which a starchy 
dent variety has been grown alongside of, or together with, a sweet 
corn. In the case where sweet corn was planted note particu- 
larly the influence of the starchy variety in modifying endosperm 
characters, and compare this with the amount of modification in 
the other variety. 

Parthenocarpy. — Examine the seedless fruits of any material 
available, such as banana, grape, navel orange. Determine 
the extent of ovular development. If English forcing cucumbers 
(such as the Telegraph) are available, cut out the stamens before 
the pollen is matured, bag the flower, and determine the effect 
upon the development of fruit in comparison with that of a fruit 
hand-pollinated at the proper time. 

References 

Church, A. H. Types of Floral Mechanism. 1:211 pp., 39 

pis., 1908. 
Correns, C. Bastarde zwischen Maisrassen, mit bes. Beriick- 

sichtigung der Xenien. Bibl. Bot. Orig.-Abh. a. d. Gesammt- 

gebiet d. Bot. 53: 161 pp., 2 pis., 1901. 
Coulter, J. M., and Chamberlain, C. J. Morphology of 

Angiosperms. 348 pp., 1903. 
Darwin, Charles. Cross and Self-fertilization in the Vege- 
table Kingdom. 469 pp., 1885. [Appleton.] 
Ewert, R. Die Parthenocarpie oder Jungfernfruchtigkeit der 

Obstbaume. 57 pp., 18 figs., 1907. 
Fletcher, S. W. Pollination in Orchards. Cornell Agl. Exp. 

Sta. Bui. 181 : 335-364, figs. 65-86, 1900. 
Klebs, G. Willkiirliche Entwickelungsanderungen bei Pflanzen. 

166 pp., 28 figs., 1903. 
Uber Variationen der Bliiten. Jahrb. f. wiss. Bot. 42 : 155- 

320, 27 figs,, 1 pi, 1905. 
Lewis, C. I., and Vincent, C. C. Pollination of the Apple. 

Oregon Agl. Exp. Sta. Bui. 104 : 40 pp., 14 pis., 1909. 
Lidforss, B. Weitere Beitrage zur Biologie des Pollens. Jahrb. 

f. wiss. Bot. 32 : 232-312, 1899. 



380 Plant Physiology 

Lidforss, B. Untersuchungen iiber die Reizbewegungen der 

Pollenschlauche. I. Der Chemotropismus. Zeit. Bot. 

1:443-496, pi. 3, 1909. 
Mobius, M. Beitrage zur Lehre von der Fortpflanzung der 

Gewachse. 212 pp., 1897. [Fischer!] 
Shamel, A. D. The Improvement of Tobacco by Breeding and 

Selection. Yearbook U. S. Dept. Agl. (1904) : 435-452. 
Waite, M. B. The Pollination of Pear Flowers. Div. Veg. 

Path, and Phys., U. S. Dept, Agl. Bui. 5: 110 pp., 12 pis., 

1894. 

Texts. Detmer, Jost, Pfcffer, Strasburger. 



CHAPTER XV 
THE SEED IN PLANT PRODUCTION 

Necessarily the quality and potential vigor of the seed 
are most important considerations in crop production. 
Quality is to a very large extent based upon physiological 
conditions. It does not seem entirely appropriate to dis- 
cuss here such matters as " true to type " seed, impurities 
and adulterants, the contamination of the seed by means 
of fungous spores present, or of a dormant mycelium within 
the tissues which may carry disease to the new crop. We 
are concerned, however, with the adaptability of the strain 
to the conditions under which it is to be grown, and with 
the capacity of the seed to produce the most vigorous 
plants of which the variety is capable. Quality of the 
seed so far as vigor and adaptability are concerned will be 
affected by conditions which permit of an arrangement in 
the. following category : — 

By the conditions under which the parent plant has been 
grown. 

By the conditions under which maturity has been attained. 
By methods of harvesting and curing. 
By the period and conditions of storage. 
By size and weight. 

226. Habitat conditions of the parent plant. — This is 
properly an hereditary consideration, but it is conven- 
381 



382 Plant Physiology 

iently treated here for special emphasis. It is now estab- 
lished beyond reasonable doubt that the quality of seeds 
will be modified in a single generation by the climate and 
cultural conditions under which the crop has been grown. 
This may have no reference whatsoever to the special 
factors influencing curing, storage, and the like. For a 
long time it has been clearly recognized that if early corn, 
notably sweet corn, is grown from the same seed at points 
North and South, there will result differences in the quality 
of the seed produced, so far as earliness is concerned, so 
that if seed from the two regions are sown side b} r side, that 
from the North will mature earlier. 

It is possible that in an indirect way the differences may 
be in some cases ultimately referred to maturity; yet 
these effects must be regarded, at present, as the imme- 
diate effects of the environment. Many data respecting 
the rapidity of the changes which may be induced under 
different cultural and climatic conditions have been given in 
agricultural literature. The table on page 340, from Lyon, 
exhibits results with a variety of wheat grown several 
years in different localities (with, of course, some oppor- 
tunity for selection) and finally in adjacent plots in Ne- 
braska, — these being strains which were, " without 
doubt, originally the same." 

The facts developed regarding corn are also true when 
applied to spring wheat and oats, for it is agreed that in 
planting spring wheat, seed obtained from farther north 
will ripen earlier and give better yield, as well as quality, 
than seed of the same strain introduced from a point 
farther south. In the case of winter wheats, however, the 
facts seem thoroughly to substantiate the general belief 



The Seed in Plant Production 



383 



that the reverse condition is true; that is, seed from points 
south give a better yield than northern grown seed of that 
strain. In general, adjusted local varieties are best. 

Modifications induced in Wheat by Its Environment 





Kansas 
Seed 


Nebraska 
Seed 


Iowa Seed 


Ohio Seed 


Date of sowing 


Sept. 9 


Sept. 9 


Sept. 9 


Sept. 9 


Lodged . . . 


None 


None 


Badly 


Badly 


Rust . . . 


Very little 


Very little 


Much 


Much 


Date of ripen- 










ing ... 


June 25 


June 27 


July 2 


July 3 


Yield of grain 










per acre . 


29.1 bu. 


27.5 bu. 


22.3 bu. 


23.1 bu. 


Weight of grain 










per bushel . 


64.2 lb. 


62.2 lb. 


56.9 lb. 


58.9 lb. 



227. Localization of seed production. — In many cases 
it is not possible to explain the localization which charac- 
terizes commercial seed production, and it would appear 
that seeds are grown in particular regions to-day, not only 
because of some apparent advantages of the region with 
respect to the maintenance or development of desirable 
hereditary qualities, but also because of such factors as 
(1) cheapness of production in the locality, (2) the effects 
of conditions upon maturity and curing, referred to later, 
or else (3) from merest accident. It would be well if 
more general recognition were given to the two underlying 
principles. On the one hand there are regions, or locali- 
ties, especially favorable for the production of high quality 
in seeds ; on the other hand, it is often the case that 
local seed production and seed selection would have 



384 Plant Physiology 

advantages outweighing all other considerations of special 
habitat. 

In the United States alfalfa seed mature well only in the 
dry climate of states like Colorado and Utah. In the 
South the potato matures so early that a long season of 
storage, resulting in probable injury, would be required if 
home-grown tubers were used in planting. 

Some growers have expressed the opinion that there 
is a marked physiological change more or less gradually 
developed in strains of onions or radishes repeatedly grown 
in certain sections of California. It is not possible at pres- 
ent to determine if other factors have been overlooked, 
but at any rate it is believed that seed from radishes which 
have been grown for successive years in California will, 
when planted in other sections of the country alongside of 
the home-grown or recently imported seed of the same strain, 
show clearly that the far western-grown product has under- 
gone some marked change with respect to eastern conditions. 

Similarly, onions grown from California seed are said 
to be different in keeping quality from those bulbs grown 
from seed produced in Michigan. This effect is said to 
assert itself even when the most stringent methods of 
selection are practiced. There is grave doubt if this is a 
general rule, and we may well believe that varieties may 
be developed which will not show this tendency. 

The seed of cabbage, cauliflower, and some other crucifers 
were first grown extensively in this country upon Long 
Island, and the region became famous for the production 
of these crops. More recently it has been found that a 
similar favorable locality is the Puget Sound region. 
Growers are so sure of the wholesome effects of these 



The Seed in Plant Production 385 

localities upon the product that one will frequently hear 
it stated that the failure to head properly is due to the fact 
that the seed was not grown in either of these regions. 

Tomato seed are grown extensively in Michigan, and they 
have been successfully produced in many parts of the North 
and of the central -West. On the other hand, the belief 
is prevalent that tomatoes grown from seed produced 
in the South rapidly deteriorate, and that in the course 
of a few years the well-established and highly prized 
varieties may revert to the common little-tomato type. 
Here again there are no statistical data indicating that 
these opinions have been formed as a result of any properly 
controlled experiments. It is, for instance, quite possible 
that by means of crossing between varieties, or by crossing 
with the little-tomato type these reversions may be ac- 
counted for. 

Tracy has reported that beans are promptly modified 
by soil conditions, and that in general seed should be grown 
on the type of soil for which they are intended. It is, 
furthermore, an interesting fact that German and French 
growers importing seed are often careful respecting the 
climatic and soil conditions under which the seed are 
grown. 

228. Maturity. — Quality may also be affected by the 
conditions which maintain just at the time the seed is 
maturing or during the state of maturity. Too much 
moisture at the time the seed is approaching this state 
precludes a proper gradual ripening, and the final effect 
is usually manifest in decreased vitality; that is, lessened 
capacity to germinate, and this is true even if the seed 
is subsequently dried and stored. The reduced vitality 



386 



Plant Physiology 



may be connected with the conditions in which the food 
substances are stored in the seed, with the development of 
injurious substances which lead to undesirable transfor- 
mations, or with the continuance of activity after the 
seed should have attained practically a dormant condi- 
tion. 

Moreover, immaturity has a tendency to lessen the 
keeping quality of most seeds, and many of the shrunken 
seeds upon the market, frequently met with in the case of 
alfalfas and clovers, are due to their immaturity at the 
time of harvesting. Apparently it is a general rule that 
the sooner immature seeds are sown the more vigorous 
will be the plants which they are able to produce. In 
other words, a gradual deterioration takes place in stor- 
age, but more promptly than in the case of well-matured 
seeds. Extensive experiments in determining the effect of 
maturity upon vitality as exhibited by germination tests 
were carried out by Hellriegel. In the case of rye the 
seeds were harvested at four different stages, and the 
following table indicates the relative condition of ripeness 
and the percentage of germination from such seeds, which 
were subsequently treated alike with respect to drying 
and storage : — 



Stage of Ripeness 


Percentage of Germination 


Contents of kernel watery .... 


4.5 
5.0 


Dough stage 

Yellow ripe 

Dry ripe 


9.5 
36.0 
84.0 



The Seed in Plant Production 



387 



The above data were secured from seeds immediately 
separated from the parent stalk and then dried. When, 
however, the seeds were allowed to remain attached to the 
harvested stalks, notable gain in the vitality was shown 
by those seeds harvested in early stages. In such experi- 
ments as the last mentioned there is opportunity for con- 
siderable ripening after the early harvesting, and the 
results are not contrary to what might be expected. 

According to the experience of some observers, a con- 
tinued practice of selecting immature seeds may result 
in the development of an earlier variety. This is some- 
times, however, at the expense of size, quality, and vitality. 

Kedzie has shown the effect of maturity upon vitality 
of wheat, and his results are so striking that they may be 
presented in detail : — 

Maturity as Affecting Vitality (Kedzie) 



Date of Harvest 



June 26 
July 4 
July 10 
July 12 



Milky juice . 
Dough . . . 
Full yellow ripe 
Dead ripe . . 



Yield 
per Acre 



11 bu. 
25 bu. 
30 bu. 

28 bu. 



Length of 
Plumule 



6.0 in. 

9.0 in. 
10.1 in. 
11.0 in. 



It should be said, however, that so far as ability to 
grow is concerned, no very narrow restrictions may be 
placed upon the stage of development of the seed, provided 
adequate and suitable nourishment can be given the young 
embryo. In a series of experiments recently carried out 
by the writer, whereby the young embryos were trans- 
ferred from the developing seeds to sterile nutrient solu- 



388 Plant Physiology 

tions, the results confirm the view that embryos thus 
treated are able to maintain themselves and sometimes able 
to develop mature plants. The vigor and strength of the 
plant, however, was in direct proportion to the degree of 
maturity of the transferred embryo. In view of these 
facts, it is unlikely that the selection of immature seeds is 
to be recommended as a means of securing earliness, unless, 
of course, all other methods fail. 

229. Conditions of harvesting and curing. — The con- 
ditions of harvesting and curing form in a measure a con- 
tinuation of the phenomenon of maturity, and a discussion 
of these factors might be included in a broad interpretation 
of the general process of maturity. However, it is a dis- 
tinct phase of the subject and deserves full, independent 
consideration in this place. Uniformly favorable con- 
ditions for harvesting and curing a given crop, other factors 
remaining fairly similar, may alone be sufficient to establish 
seed production as an industry in a locality. 

Among the most striking instances of localization which 
are to be found is that of the Santa Clara Valley, California. 
This region has won an enviable reputation for seed- 
growing, and over numerous other equally fertile localities 
it possesses the distinct advantage of relative certainty 
in the prevalence of uninterrupted dry conditions during 
late summer and far into the autumn, the time when most 
seeds are harvested. Hundreds of acres of the sweet-pea 
are grown for seed in California, yet the sweet-pea is 
equally thrifty and vigorous in many other sections of 
the country. Dry summers and autumns are particularly 
important, moreover, in cases where, in order to harvest 
the seed, the whole crop must be cut, and there results 



The Seed in Plant Production 389 

consequently a large bulk of material which must be cured 
previous to threshing. 

In harvesting and storing seed the unfortunate practice 
often prevails of storing the product in bulk; this in spite 
of the fact that no small proportion may be somewhat 
immature. As a common result, the material heats rapidly, 
and in the end much loss may occur (see section 168). 
This heating is due in many cases to respiration, yet a part 
of the difficulty also lies in the fact that the growth of 
microorganisms is much encouraged by the "sweating 
process." 

230. Duration of vitality. — In recent years considerable 
attention has been bestowed upon the problems of main- 
tenance of seed vitality, and upon a determination of the 
conditions which are injurious. Much new work and 
valuable data are therefore available; but the problem 
is not a new one, and much was done by De Candolle and 
others fully eighty years ago. 

Species differ in a decided manner with respect to the 
length of time in which vitality is maintained, and this is 
true whether the conditions to which they are subjected 
are favorable or unfavorable. Among seeds readily killed 
by storage for a relatively short period may be included 
those of many Composite, Cruciferse, and Gramineae; 
rfvhile some of those far more resistant are Malvaceae, 
Solanaceae, hard-seeded Leguminosae, and in general those 
with water or air-resistant seed-coats. It should not be 
understood, however, that all species of the same genus 
or family are even approximately alike in resistance. 
Becquerel reports an age of about eighty years for several 
species of legumes which were still capable of germination. 



390 



Plant Physiology 



The following table from Duvel indicates the average loss 
of vitality of thirteen kinds of seed sent to seven different 
localities in the United States and to Porto Rico, and 
kept under ordinary conditions of storage, the first test 
covering a period averaging 128 days (February to June), 
the second period averaging 251 days (February to Octo- 
ber) : — 







First Test 


Second Test 




Kind of Seed 








Deterioration in 


Deterioration in 






Vitality 


Vitality 






Per cent 


Per cent 






2.55 
3.92 


5.20 


Pea . . . 




11.39 


Corn, sweet 


'A" 


1.20 


12.17 


Watermelon 




.57 


12.51 






1.96 
11.02 


15.77 


Radish . 




22.67 


Corn, sweet 


'B" 


12.47 


26.10 






5.76 
7.22 


29.58 


Cabbage 




43.56 


Carrot . . 




9.77 


53.89 


Onion . . 




15.26 


74.10 


Pansy . . 




38.33 


84.90 


Phlox Drummondii 


34.97 


85.85 



The control seeds referred to in the table were kept in 
a cool, dry closet in the botanical laboratory, Ann Arbor, 
Mich., and these showed a remarkable vitality. 

231. Environmental conditions. — Duvel determined 
that under ordinary conditions moisture and temperature 
are the more important factors. Rise of temperature 
alone may not be injurious unless accompanied by in- 



The Seed in Plant Production 



391 



creased moisture-content. The following table gives a 
record of vitality as related to precipitation and tempera- 
ture at the seven points in the United States where the 
thirteen kinds of seed were stored : — 



Place where Seeds 


Average 

Loss OF 

Vitality 

13 Kinds of 

Seed 


Annual 
Precipita- 
tion 


Temperature 


were Stored 


Mean Fahr. 


Maximum 
Fahr. 


Mobile, Ala. . . . 
Baton Rouge, La. 
Durham, N.H. . . 
Auburn, Ala. 
Lake City, Fla. 
Wagoner, Ind. Ter. . 
Ann Arbor, Mich. 


Per cent 
71.98 
41.39 
39.58 
33.91 
29.38 
28.41 
2.52 


Inches 
91.18 
66.37 
48.20 
62.61 
49.76 
42.40 
28.58 


Degrees 
71.4 
72.2 
52.3 
64.4 
73.3 
67.1 
49.12 


Degrees 

96 

98 

98 

98 
103 
107 

98 



In general, it would seem that a further drying out of 
thoroughly matured seeds may enhance the keeping ca- 
pacity. Moreover, such mature seeds keep well at high 
temperatures. Immature seeds, or those which may not 
be thoroughly dried out, keep best in a cool, dry situation. 
When moisture is present, it would seem that respiration 
is rapid and may be regarded as an important factor in 
reducing vitality. Under ordinary conditions " the life 
of a seed is undoubtedly dependent on many factors, 
but the one important factor governing the longevity of 
good seed is dryness." 

232. Buried seed. — Duvel, Beal, and others have 
shown that, in general, seeds which are buried deeply 
maintain their vitality for a long period. An instance 



392 Plant Physiology 

came to my attention in Columbia, Mo., of the germina- 
tion of clover seed which had been buried for more than 
thirty years. The conditions were these : A cut was made 
in clay soil exposing the ground-level of a fill made more 
than thirty years before of from two to four feet. A few 
weeks after the exposure of the old ground-level a contin- 
uous growth of white clover appeared along that line. 
There could be no doubt of the age of those seed, and an ex- 
amination of undisturbed soil farther in disclosed the fact 
that there were present not only white and red clover 
seed capable of germination, but also, in smaller quantity, 
cocklebur, sonchus, and a species of sedge. 

In general, it would seem that the burial of agricultural 
seeds results in death far more promptly than in the case 
of resistant weed seeds. Shallow burial of weed seeds, 
however, affording moisture conditions favorable for 
decay, may often result in their destruction. 

233. Delayed germination. — The rest period of the 
seed seems to be to a considerable extent, if not entirely, 
due to the development of a structure, or device, during 
the maturing process which may serve to exclude water 
or air until acted upon by gradual processes of decay or 
special agents. It is well known that the germination of 
many seeds is quickened by soaking in strong sulfuric 
acid, by cracking the tough seed-coats, and some even by 
the action of the digestive juices of certain animals. 

Nobbe and others have pointed out the relation of 
germination to certain structural devices. Recently 
Crocker finds that the marked case of delayed germina- 
tion in the seed of Abutilon is due to the fact that the 
condition of the seed-coats precludes the possibility of 



The Seed in Plant Production 393 

water absorption. Again, in the case of a few seeds, it 
seems to be established that the exclusion of oxygen is 
the important factor. The resting spores of many fungi 
are notably difficult to germinate until after a period of 
rest, and it is quite probable that similar factors are con- 
cerned here, especially through the deposition of some 
resinous substance in the cell-wall. 

234. Effect of weight and size of seed upon vigor. — 
Since the weight and size of seed determine the amount 
of food-material immediately available for the plantlet, 
at the time of germination, it is to be inferred that these 
factors might have some influence upon production. 
Early experiments by Hellriegel, Wollny, Marek, and 
others were favorable to the view that seed of greater 
size and weight give generally more vigorous plants than 
those smaller or lighter. Much additional experimental 
work has been reported in recent years, and some of this 
evidence should be considered with respect to a few crops. 

The problem is not so simple as it seems. Viewing the 
matter from the standpoint of the factors readily recog- 
nized, the effect of the accumulated food-materials is cer- 
tainly to start the seedling off vigorously. If the coty- 
ledons of the bean or pea are removed even during the 
late stages of germination, the plants thus deprived of a 
portion of their resources fall behind in growth. It is to 
be expected that the final effect of this loss would depend 
much upon the conditions subsequently encountered. If 
the season is bad, or the soil poor, the seedling with more 
potentiality in itself should be able to become established 
more safely and quickly, and the advantage secured 
might persist. Hellriegel supports the view that differ- 



394 



Plant Physiology 



ences at maturity between the product of heavy and light 
seed are intensified when the conditions are unfavorable. 
With all conditions favorable, differences at first evident 
might, in time, disappear. In all cases, comparisons are 
only fair within the variety. 

Hicks and Dabney have made a test of the relative 
effects of weight upon vigor, using many sorts of seeds. 
They attempted to eliminate all unsound seed, conse- 
quently the material was sieved and afterwards hand 
selected. The results are as follows : — 

Experiments with Heavy and Light Seeds 



Name and 
Variety 


Number 

of Seeds 

in Each 

Lot 


Weigh r 
of Seeds 1 


Number 
Germi- 
nated 


Number 
of Plants 

WEIGHED 

in t Each 
Lot 


Number 
of Days 
of Ex- 
periment 


Weight 

of Seed- 
lings 






Grams 








Grams 


Radish, early 


1 f 

| 100 -. 


A 1.770 


A 73 


1 

} 58 
J 


24 j 


A 49.5 


long scarlet . 


B 1.037 


B 84 


B 31.5 


Vetch, winter . 


50 


A 4.077 
B 2.099 


A 48 
B 47 


I 47 

J 


15 j 


A 33.0 
B 18.0 


Sweet pea, Her 


| 50 ] 


A 6.092 


A 46 


1 

• 41 


26 


A 58.0 


Majesty . . 


C 4.045 


C 47 


C 44.4 


Cane, Early 


I 10 ° 1 


.4 2.411 


A 43 


] 


40 ] 


A 23.5 


Amber . . . 


B 1.360 


B 48 


B 12.0 


Kafir Corn, red . 


100 


A 3.298 
B 1.741 


A 90 
B 49 


! 47 

J 


39 1 


A 22.0 
B 13.0 


Rye, University 
of Minnesota, 
No. 2 . . . 


50 


A 1.105 
B .745 


A 45 
B 45 


1 

[ 45 

J 


23 


A 34.5 
B 20.0 


Oats, White 


1 .0 ! 


A 1.298 


A 50 


| 49 


23 


A 37.2 


Wonder . . 


j i 


B .805 


B 49 




B 25.0 



1 A, heavy; B, lighter than .4; C, lighter than B. 



The Seed in Plant Production 



395 



From these results it seems just to conclude that, in gen- 
eral, a more vigorous growth, and consequently a better stand 
in the field, is secured by employing only the heavier seed. 

235. Experiments with wheat. — The effect of size of 
seed on production has been with no other plant so exten- 
sively studied as with wheat. The evidence is most con- 
tradictory. The majority of the results seem to favor 
the view that large or heavy seed are preferable, especially 
when among the small seed are included distinctly imma- 
ture grains. With wheat the factors are complex, for 
size may be considerably affected by plumpness, and the 
latter may be due largely to starch and water content. 
Additional starch in the grain may not affect the vigor 
and yield of the plant secured from such seed. Again, 
in the same variety, there may be different types or strains, 
— some with larger grains, some with smaller, although 
the yields may run practically the same. All these factors 
may affect the experiments. The results of grading and 
testing seed wheat are shown in subsequent tables. 

In the first case reported by Zavitz, the seed were 
selected from both winter and spring wheats, and the 
experiments were continued five and eight years, respec- 
tively, but each crop was grown from previously unse- 
lected seed : — 





Kind of Seed 


Yield per Acre in Bushels 


Spring Wheat 


Winter Wheat 


Large, plump 

Small, plump 

Shrunken 


21.7 
18.0 
16.7 


42.4 
34.8 
33.7 







396 



Plant Physiology 



In the second case, reported by Hickman, to be con- 
trasted with the preceding, " three grades were used: 
first grade, the large grains ; second grade, the best of the 
grains passing through the sieve in screening out the first 
grade ; third, unscreened wheat as it came from the 
thresher." The experiments were continued nine years, 
and after the first year, the selections for each were made 
from the same grade of the previous year : — 





Yield for 9 Years, Bu. per Acre 


Average Weight per Bushel 


First Grade 


Second Grade 


Third Grade 


First 
Grade 


Second 

Grade 


Third 
Grade 


15.48 
17.16 
16.11 


16.06 
17.64 
15.82 


16.03 
16.69 
16.06 


57.8 
52.2 
57.7 


58.3 
57.9 
58.0 


58.1 
58.1 
57.4 


16.25 


16.50 


16.26 


57.6 


58.1 


57.9 





There is also difference of experience with respect to 
large and small grains from the same head or plant. 

236. Experiments with cotton. — Comparative pro- 
duction tests of the value of heavy cotton seed over the 
usual farm product have been made at the United States 
Department of Agriculture. 1 The heavy seed were sepa- 
rated, in the case of those varieties the seed of which are 
covered with fuzz, by special devices and methods. The 
field tests were made with results as shown in the table 
on the next page. 

1 Webber, H. J., and Boykin, E. B., " The Advantages of Planting 
Heavy Cotton Seed." U. S. Dept. Agl., Farmers' Bui. 285 : 16 pp., 6 
figs., 1907. 



The Seed in Plant Production 



397 



Variety, Hawkins, Test at 


Yield in Pounds on Equal Areas, Each 
approximately One Acre 


Lamar, S.C. 


First 
Picking 


Second 
Picking 


Third 

Picking 


Total 

Yield 


Heavy seed (20 rows) .... 
Unseparated seed (20 rows) . 


375 
335 


253i 

228 


419 
3811 


10471- 
944i 


Variety, Jones's Improved, Test 
at Hartsville, S.C. 










Heavy seed (14 rows) .... 
Unseparated seed (14 rows) . 


1581 
139 


793 

7151 


2121 
22H 


11641 
10751 



In these two cases, the gain from the use of heavy seed is 
respectively 10.9 and 8.25 per cent. This is by no means 
a trifling gain when reckoned as additional profit per acre. 

237. Experiments with tobacco. — Among practically 
all varieties of tobacco there is great difference in the size 




Fig. 108. Tobacco plants from seeds of different sizes ; heavy, medium, 
and light seeds respectively employed, from right to left. [Photograph 
from Bureau of Plant Industry.] 



398 Plant Physiology 

and weight of the seed from similar individuals. Trabut 1 
found it possible to effect a separation into heavy and 
light sorts through the capacity of these two kinds, re- 
spectively, to sink or float in water. It was found that the 
heavy seed produced plants which were greener, more 
vigorous, and of larger size. Shamel has made further 
studies of this relation, separating, by means of a current 
of air, the seeds into three categories — heavy, medium, 
and light. Samples of these seed were germinated, and 
the accompanying illustration shows the relative vigor of 
the plants resulting from the different grades. 

LABORATORY OR SUPPLEMENTARY WORK 

Write a report upon the vitality of seed as affected by 
methods of harvesting and storage, consulting the literature 
accompanying this chapter, also such of that contained in re- 
cent volumes of the Experiment Station Record as may be 
readily available. 

References 

Crocker, W. Role of Seed-coats in Delayed Germination. 

Bot. Gaz. 42 : 265-291, 1906. 
De Candolle, A. P. Physiologie vegetale. 2 : 618, 1832. 
Detmer, W. Vergleichende Keimungsphysiologie. 565 pp., 

1880. 
Duvel, J. W. T. The Vitality and Germination of Seeds. 

Bur. Plant Ind. U. S. Dept. Agl. Bui. 58 : 96 pp., 1904. 
■ The Vitality of Buried Seeds. Bur. Plant Ind. U. S. Dept. 

Agl. Bui. 83 : 22 pp., 3 pis., 1905. 
Hickman, J. Fremont. Field Experiments with Wheat. Ohio 

Agl. Exp. Sta. Bui. 129 : 27 pp., 1901. 

1 Trabut, L., Bui. 17, Service Botanique de l'Algerie. 



The Seed in Plant Production 399 

Hopkins, C. G. The Chemistry of the Corn Kernel. 111. Agl. 

Exp. Sta. Bui. 53 : 1898. 
Lyon, T. L. Improving the Quality of Wheat. Bur. of Plant 

Ind., U. S. Dept. of Agl. Bui. 78: 120 pp., 1904. 
Modifications in Cereal Crops induced by Changes in 

Their Environment. Proc. Soc. Prom. Agl. Science. 28: 

144-163, 1907. 
Nobbe, F. Handbuch der Samenkunde. 631 pp., 339 figs., 

1876. 
Pieters, A. J., and Brown, Edgar. Kentucky Blue Grass 

Seed : Harvesting, Curing, and Cleaning. Bur. of Plant 

Ind., U. S. Dept. of Agl. Bui. 19: 19 pp., 6 pis., 1902. 
Shamel, A. D. The Improvement of Tobacco by Breeding and 

Selection. Yearbook U. S. Dept. Agl. (1904), 435-452. 

(Value of large and heavy seed, pp. 440-442.) 



CHAPTER XVI 
THE TEMPERATURE RELATION 

A large number of species of plants composing the 
main vegetation of the earth are seldom, if ever, exposed 
within their normal ranges to great extremes of tempera- 
ture. There are many annuals which first appear after 
the dangers of severe frosts are past, and they perfect 
their fruits long before the growing season is closed. A 
considerable number of perennials may be exposed to ex- 
tremes only in a resting or semidormant condition. In 
general, then, native plants have been long acted upon 
by the particular climatic factors of the region, so that 
they show in a telling manner the influence of a long line 
of ancestry whose development and survival within the 
region is at least relatively fixed. 

238. Climatic extremes and introduced plants. — In- 
troduced plants in any region are, generally speaking, 
much more likely to suffer exposure to an injurious ex- 
treme, especially cold ; yet exceptional conditions may 
bring disaster to any type of vegetation. The peach in 
the South and Southwest is sometimes in blossom before 
the winter is at an end, and the blossoms are not infre- 
quently caught by late frosts. The famous peach belt of 
Michigan was visited in 1905 by an early frost in October, 
and the result was the practical annihilation of the peach 
400 



The Temperature Relation 401 

industry in that section, for the wood of the peach trees 
was entirely " unripened." 

Throughout a large portion of the zone of its culture the 
cotton plant on well-watered and rich land grows contin- 
uously until killed by frosts. In the same way the nas- 
turtium and the tomato may be in full growth when killed 
by frost. To a less extent this is true for familiar native 
plants of the field. In spite of these facts, the impression 
should not prevail that the vegetative period of a plant is 
so fixed by heredity and ancestral adjustment as to be 
incapable of responding fairly rapidly to the new environ- 
ment. In a new region the growing season of a species or 
variety may be changed noticeably within a very few years. 
Corn from the far South with a growing period of six 
months will, if at all able to maintain itself in the North, 
modify its period of growth so that it will mature well 
within the season. Relatively few crops, however, are 
able to survive and propagate themselves if left to form 
fruit and germinate in the open, and in the relation of 
cultivated crops to temperature the question is more 
complex than is generally assumed. 

239. Temperature and production. — As one goes north- 
ward in the United States or in Europe, a certain general 
change of crops is evident, indicating the universal im- 
portance of the temperature factor in modifying produc- 
tion. Potatoes may be grown from Mexico to Maine, but 
throughout this whole range the growing season is well 
within the normal length of the Maine summer. In fact, 
in the far South two crops may be grown during a single 
season. Corn is produced in the same region, but certain 
strains of field corn grown in the South might not reach 



402 Plant Physiology 

maturity unless protected during the first season in New 
England. The cotton and the cowpea disappear entirely 
in a little more than half the range of corn, while timothy 
and barley, almost unknown southward, approach their 
prime near the northern limit of this area. 

In any scheme of continental plant zones, temperature is 
recognized as most important. In general, such zones are, 
therefore, constructed with special reference to the annual 
or seasonal isotherms. No scheme of regions based largely 
upon a single factor is entirely satisfactory. It is better, 
however, than no attempt at classification. Koeppen, 
Schimper, and others have indicated, on a broad basis, the 
plant zones of the earth, and Merriam has arranged for 
North America a suggestive scheme of life and crop zones 
(Fig. 2). 

240. Cardinal temperatures. — Certain cardinal temper- 
atures are recognized. " Maximum " and " minimum " 
are terms referring respectively to the highest and lowest 
temperatures at which the development of a particular 
organism may occur. It is apparent, however, that there 
may be separate maxima and minima for ever}' process or 
activity of the plant. The maximum temperature for 
germination may be below that which will support con- 
tinued growth in the developing plant. It is difficult, or 
at least inconvenient, to determine the most favorable 
temperature for any process or function : yet, within cer- 
tain limits, such determinations are possible. The most 
favorable temperature is designated the optimum. It is 
also customary to employ the terms " ultra-maximum" and 
" ultra-minimum/' denoting respectively the death point at 
high and at low temperature. The following tables from 



The Temperature Relation 



403 



Haberlanclt give a comparative view of the relation of 
some familiar plants to these cardinal temperatures : — 

Cardinal Temperatures for Growth, Degrees C 



Buckwheat 
Hemp . . 
Oats 

Rye . . 
Rape . . 
Wheat . . 
Barley . . 
Flax . . 
Pea . . . 
Sunflower . 
Maize . . 
Pumpkin . 
Tobacco . 
Melon . . 
Cucumber . 



0-4.8 

0-4.8 

0-4.8 

0-4.8 

0-4.8 

0-4.8 

0-4.8 

0-4.8 

0-4.8 

4.8-10.5 

4.8-10.5 

10.5-15.6 

10.5-15.6 

15.6-18.5 

15.6-18.5 



25-31 
37-44 
25-31 

25-31 

25-31 
25-31 

25-31 
31-37 
37-44 
37-44 

31-37 
31-37 



37-44 
44-50 
31-37 
31-37 

31-37 
31-37 

31-37 
37-44 
44-50 
44-50 

44-50 

44-50 



Cardinal Temperatures for Germination, Degrees C. 



Zea Mays .... 
Phaseolus multiflorus 
Cucurbito Pepo . . 
Triticum vulgare 
Hordeum .... 



9.4 
9.4 
14.0 
5.0 
5.0 



34.0 
34.0 
34.0 
29.0 
29.0 



46.2 
46.2 
46.2 
42.5 
37.5 



These figures should be considered as merely suggestive, 
for it is apparent that differences in varieties, in local ad- 



404 Plant Physiology 

justment, and also in environmental factors will affect 
the cardinal temperatures in any particular case. 

Reference has been made already to the fact that pho- 
tosynthesis, metabolism, and other processes or responses 
of the plant are to a certain point rapidly accentuated 
with increase of temperature. Blackman has shown very 
clearly that maximum activity, especially for respiration 
and photosynthesis, has commonly been placed too high, 
since proper consideration of the time factor has not al- 
ways been given. 

241. Inhibition at high temperatures. — From recent 
work reported by Balls it would seem that the inhibition 
of growth at high temperatures during a considerable pe- 
riod of time is in all probability the result of an accumula- 
tion in the cells of injurious metabolic products. The time 
factor is most important. According to his views, some 
of these deleterious products are produced at low tempera- 
tures, but under such circumstances they are constantly 
decomposed, whereas at high temperatures production is 
more rapid, and consequently accumulation and injury 
result. Upon this hypothesis the effect of high tem- 
perature upon the protoplasm would be that of favoring 
auto-intoxication. 

242. Heat units. — Considerable attention has been be- 
stowed upon computations of the heat units (thermal con- 
stants) required to mature certain crops. Such data are 
not without interest, yet examination of the evidence thus 
far accumulated indicates that there is practically no such 
thing as a relatively invariable thermal constant for any 
plant when factors other than temperature are inconstant 
or uncontrolled. Assuming that every other factor of the 



The Temperature Relation 



405 



environment is constant, there is a theoretical thermal con- 
stant, and it is of sufficient importance to receive some 
practical consideration. 1 

243. Heat units and germination. — If the number of 
heat units required in order to bring a plant to maturity 
were at all constant, then the number requisite for any 
phase of growth should likewise be more or less constant. 
Some interesting data are available respecting germination, 
and in the following table the time intervals are given for 
germination at the temperatures indicated, and the heat 
units may be readily computed : — 



Sinapis alba 


Linum 
usitatissimum 


Melon 
(Cantaloupe) 


Trifolium 
repens 


Zea Mays 


Temp. 
°C. 


Time 


Temp. 
°C. 


Time 


Temp. 
°C. 


Time 


Temp. 
°C. 


Time 


Temp. 
°C. 


Time 


0.0 


408 


1.8 


816 


16.9 


222 


5.7 


240 


9.2 


240-288 


1.9 


384 






19.4 


68 


9.2 


144 


12.9 


120-168 






4.8 


408 


25.05 


44 


12.9 


72 


16.9 


90 


5.7 


96 


5.7 


144 


28.0 


74.4 


13.0 


69 


21.1 


42 


9.2 


84 






40.6 


94 


17.05 


62.4 


25.05 


23-44 


12.9 


41 


12.9 


66 






21.1 


42 


28.0 


36-48 


17.2 


41 










25.05 


42 






21.1 


22 


17.05 


72 






28.0 


72 






25.05 


36 


21.1 


36 






34.0 


192 






28.0 


72-78 


25.05 

28.0 
34.0 


38 

60-72 

192 















1 There are several methods of computing heat units. In each case it 
is necessary to know the period of growth in days and the daily mean 
temperature during the growing period. With this data we may then 
obtain the total heat units by multiplying the growth period by the daily 
mean temperature. This method makes 0° C. or 32° F. the basis. In the 



406 



Plant Physiology 



Sum of Daily Mean Temperatures above 18° C. (64.4° F.) for 
Fruiting Period of Date-palm from May 1 to Oct. 31 



Locality 


Sum of Daily 
Mean Temp, 
above 18° C. 

(64.4° F.) 


Remarks 




Degrees 
C. 


Degrees 
F. 


Meteor- 
ological 


Ripening 


Algiers, Algeria 
Orleansville, Algeria . . 
Fresno, Cal 

Tucson, Ariz 

Cairo, Egypt .... 
Phoenix, Ariz. (Salt River 

Valley) 

Biskra, Algeria . . . 

Ayata, Algeria (Oued Rirh 

region) 

Tougourt, Algeria 
Bagdad, Mesopotamia . 

Indio, Cal. (Salton 

Basin) 

Salton, Cal 


652 

788 
1,054 

1,409 

1,593 

1,677 

1,836 

1,906 

2,049 
2,356 

2,237 

2,679 


1.174 
1,418 
1,897 

2,538 

2,868 
3,019 

3,304 

3,431 

3,689 
4,242 

4,027 

4,823 


Obs. 

6 yrs. 

Temp. 
1891 

Obs. 

7 yrs. 
Obs. 
12 yrs. 


No dates ripen. 

Very early sorts mature. 

Sorts grown usually 
fail to ripen. 

Sorts now grown usu- 
ally fail to ripen. 

Dates ripen regularly. 

Many sorts ripen reg- 
ularly. 

Date culture the lead- 
ing industry. Even 
Deglet Noor ripen. 

Deglet Noor dates ripen, 
but not always well. 
Do. 

Ma ny excellent varieties 
ripen. 



case of the F. scale it is necessary, of course, to subtract 32 from the daily 
mean before multiplying for the product. It would seem, however, that 
the method of computation to be preferred is one whereby an approxi- 
mate growth minimum is taken as the basis, and the difference between 
this and the daily mean represents the daily efficiency during the grow- 
ing period. An example in the latter case is as follows : assuming the 
growth period of wheat to be 100 days, the minimum growth temperature 
40° F., and the daily mean to be 70° F., we have 70 — 40 x 100 = 3000° F. 



The Temperature Relation 407 

244. The date-palm. — One of the most important ap- 
plications of a study of the relation of plants to the heat 
units of the region in which they are grown is that made by 
Swingle respecting the date-palm. He has shown that as 
the heat units increase, the general adaptability of arid 
regions to date culture is advanced, and a certain minimum 
may not be exceeded for any type of date. From such a 
study it was considered possible to foretell with approxi- 
mate accuracy what section of the Southwest might be 
utilized in date culture. 

245. Control of temperature. — It is obvious that limi- 
tations of expense impose pronounced restrictions upon 
the exercise of control over the temperature factor in the 
open. With a few intensive crops, such as asparagus, 
waste steam has been utilized to some extent in forcing in 
open culture, but proper control of temperature for forcing 
or for producing crops out of season is usuahy confined to 
greenhouse and hot-bed culture. 

In some sections of the United States the loss of the 
entire peach, apple, or other fruit crop may occur in conse- 
quence of one or two late frosts, when, as experience has 
shown, the temperature may fall from 4 to 14° below 
freezing. 1 Recently a control or prevention of this loss 
has been successfully accomplished by means of coal or 
oil heaters. The general plan is to place from 60 to 100 
small ovens or heaters per acre at appropriate distances 
apart. Then, if by midnight the indications are that a 
freezing temperature will be reached in the early hours of 

1 Paddock, W., and Whipple, O. B., " Fruit-Growing in Arid Regions." 
(Frost Injuries, Secondary Bloom, and Frost Protection.) Chapter 19: 
324-354, 1910. 



408 Plant Physiology 

morning, usually the coldest period of the day, the heaters 
are lighted. It has been found possible at an average cost 
of about $20 per acre to raise the temperature of the or- 
chard as much as from 5 to 14° F. above that of the nor- 
mal air, and this often in the face of considerable wind. 
The practice has recently assumed unexpected impor- 
tance, and seems to have superseded the relatively ineffec- 
tive smudge methods. 

246. The temperature of the plant. — The temperature 
of the plant is in general the temperature of the environ- 
ment. Twigs, branches, and even trunks of trees will show 
during cold weather changes of temperature more or less 
in accordance with that of the air. In the case of large 
branches or trunks some time will be required in order that 
the minimum of the air may be registered by the tree, and 
there will be, therefore, a very definite temperature lag. 

In the sunshine dark buds, branches, or trunks may ab- 
sorb heat to such an extent that the internal temperature 
will be greater than the external. In the same way, green 
leaves exposed to sunlight show a temperature from two or 
three to fifteen degrees higher than the air, depending upon 
the intensity of the light. This latter point has received 
careful attention by Blackman, who has employed in the 
work very delicate electro-thermometric methods. The 
ordinary method of wrapping the bulb of a thermometer 
with one or more thicknesses of a leaf will not afford ac- 
curate indications of the actual leaf temperature. 

247. Adjustment of structure. — There are few or no 
protective structures in plants which are of direct service 
against injurious temperatures. As will be shown later, 
both high and low temperatures act upon the plant cell to 



The Temperature Relation 



409 



cause drying out, and the structures which are ordinarily 
assumed to be protective against cold or heat are in reality 
serviceable in preventing loss of water. The delicate 
young buds of the peach or other deciduous trees may be 
inclosed by bud-scales, hairs, and resins ; nevertheless, 
such buds promptly freeze solid when the temperature 
falls below the freezing-point of the cell-sap, or the point of 
supercooling. The trunk of the tree is, of course, pro- 
tected in a way by thick bark, yet so far as the entrance of 
cold or loss of heat is concerned this protection is insignifi- 
cant. 

248. Irritable response. — Through growth movements 
toward or away from a source of heat, plants commonly 
exhibit the capacity for irritable response (positive and 
negative thermotropism) with respect to temperature ; but 
this response is of 
little practical sig- 
nificance, except as 
further evidence of 
the paratonic rela- 
tions of the organism. 
Thermonastic move- 
ments also occur, but 
this general class of 
phenomena is dis- 
cussed in section 306. 

249. Freezing. — 
Some of the results 
of freezing deserve 
careful consideration. FlG - 109 - Froze ° ^ en > °l Fli ^ Tia ''^ w ' 

. ing ice-masses (stippled). |Aiter Muiler- 

It is well known that Thurgau.] 




410 



Plant Physiology 



in the freezing of a plant cell under ordinary conditions, 
the ice crystals are formed upon the surfaces of the cells. 
In the case of tissues with intercellular spaces these crystals 
form in the latter. In this way the protoplasm gives up its 
water and the mechanical injuries of the ice crystal are not 
ordinarily exhibited within the protoplast. In the case of 
very rapid supercooling of large cells it is probable that ice 
crystals develop within the cell ; thus mechanical harm 
may result. Similarly, in tissues mechanical injury may 
sometimes result, and the bark of immature wood may be 
ruptured when severely frozen. It has been found, how- 
ever, that the diameter of a frozen twig is usually less than 
normal. 

In view of all the facts which have been presented by 
various investigators, it would appear that the ability of a 

plant to withstand cold is in 
large part determined by the 
capacity of the cells to give 
up water without injury 
during freezing. On the 
other hand, according to the 
views of Molisch, death from 
cold commonly results dur- 
ing the process of freezing. 
This refers particularly to 
active cells, or herbaceous 
shoots, and is at variance with the popular impression that 
frozen plants are less injured when thawed out gradually. 
Many plants are injured at temperatures above the 
freezing-point. This may be due to a simple disturbance 
of the water relation, but it is more probable that there 




Fig. 110. Frozen leaf-stalk of La v- 
atera, showing ice-masses (black). 
[After Muller-Thurgau.] 



The Temperature Relation 



411 



are complex effects, the permeability of the protoplasm 
being also affected. 

250. Buds. — The relation of buds to cold has received 
careful attention by Wiegand. He finds that ice may form in 
a large number of species when the temperature falls as low 







Fig. 111. Section of a bud of Popidus nigra frozen at 5° F. sectioned and 
photographed in the open ; light areas are ice crystals. [After 
Wiegand.] 

as — 18° C. At this temperature it may be formed in large 
quantities and is more abundant in cortical and paren- 
chymatous tissues than in meristem. When absent at this 
temperature, it may be assumed that the tissue is made up 
of very small cells with thick walls and low water-content. 
This is explained by the fact that " the degree of cold nec- 
essary to cause the separation of ice is proportional to the 



412 



Plant Physiology 



force which holds water in the tissue. This, in turn, de- 
pends upon the relative proportion of water to cell-wall 
and protoplasm." Measurements were made by Wiegand 
of seven species of trees frozen at a temperature of — 18° 
C. and of seven species which failed to freeze. The com- 
parative data for two species in each group are presented 
by the following table : — 



Cell Diam. 
in MM. 



Max. 
Aver. 



Min. 

Aver. 



Texture 
of Wall 



Per 
Cent 



A. Ice abundant in leaves and growing 

points at — 18° C. 

Crataegus punctata 

Prunus serotina 

B. Ice not present at — 18° C. 

Quercus alba 

Carya alba 



0.040 
0.021 



0.015 
0.048 



0.012 
0.015 



0.006 
0.015 



thin 
thin 



thick 
very thick 



49.4 
47.6 



22.7 
31.4 



LABORATORY WORK 



Effect of heat and cold upon germination. — Expose selected 
dry seed of barley or peas for 1 hour to dry-oven temperatures 
of 50° C, 75° C, and 100° C. Place 25 of each lot in a ger- 
minator together with an equal number of control seed and 
determine the effect upon the percentage of germination. In 
the same way employ seeds of the above plants in water at the 
temperatures above given, and test similarly. For further 
comparison it is also desirable to employ in both experiments 
a lot of seed which are just beginning to germinate. Discuss 
the results. 



The Temperature Relation 



• 413 



Soak some seed of barley or peas an hour or two in water 
and keep another lot dry. Expose lots of 25, soaked and un- 
soaked, to such low temperatures as may be conveniently 
prepared by freezing mixtures of salt and ice. One lot may be 
placed in test-tubes immersed in broken ice, another in similar 
tubes at from 5 to 10° below zero, and another exposed to about 
— 20° C. ; from — 5 to — 10° C. may be obtained in a freez- 



" i^t*^^ 5 ^ ^fi 




Fig. 112. Thermograph. [Illustration from Julien P. Friez.] 

ing mixture of 10 parts common salt to 100 parts snow, while 
— 20° C. requires 33 parts salt. Subsequently, test the ger- 
mination and discuss the results. 

Formation of ice crystals. — Place filaments of Spirogyra in a 
drop of olive oil in a hanging-drop culture. Expose in a cham- 
ber surrounded by a freezing mixture such that the tempera- 
ture of the chamber is reduced to about — 10° C, then remove 
the culture and examine promptly under the microscope to 
locate position of any ice crystals formed. 

On a day when the temperature of the air is about 0° C, 
or below, make sections of artificially or naturally frozen buds 
and locate the ice crystals. 

Effects upon root elongation. — By means of the method em- 
ployed in the study of growth, mark with parallel lines on the 



414 Plant Physiology 

root-tips of germinating beans, and study the effect upon elonga- 
tion of such different temperatures as may be obtained in the 
incubators at hand, in the refrigerator, and at the room tem- 
perature. In all cases the beans employed should be uniform, 
and the experiment should be carried out in a moist atmosphere. 
Temperature relations. — In case time has not permitted, in 
the appropriate place in connection with various phenomena 
discussed, to experiment upon the effects of changes of tempera- 
ture, such experiments should be included here, as far as pos- 
sible ; especially important are the effects of temperature upon 
transpiration, photosynthesis, enzyme action, and growth. 

References 

Balls, W. L. Temperature and Growth. Ann. Bot. 22 : 

557-592, 11 figs., 1908. 
Blackman, F. F. Optima and Limiting Factors. Ann. of Bot. 

19 : pp. 281-295, 2 figs., 1905. 
Davenport, C. B. Action of Heat upon Protoplasm. Experi- 
mental Morphology. 1: pp. 219-273, 8 figs., 1897; ibid. 

2 : pp. 450-469, 5 figs., 1899. 
Haberlandt, F. Die oberen und unteren Temperaturgrenzen 

fur die Keimung der wichtigeren landw. Samereien. Landw. 

Versuchsstation. 17:104-116, 1874. 
Merriam, C. H. Life Zones and Crop Zones of the United 

States. Div. Biol. Survey Bui. 10: 79 pp., 1 map, 1898. 
Molisch, H. L T ntersuchungen ueber das Erfrieren der Pflanzen. 

73 pp., 1897. 
Mueller-Thurgac, H. Ueber das Gefrieren und Erfrieren 

der Pflanzen. Landw. Jahrb. (1880): pp. 133-189; ibid. 

(1886) : pp. 453-609. 
Swingle, Walter T. The Date Palm and Its Utilization in the 

Southwestern States. U. S. Dept. of Agl. Bur. of Plant Ind. 

Bui. 53 : 155 pp., 22 pis., 10 figs., 1904. 
Wiegand, K. M. Some Studies regarding the Biology of Buds 

and Twigs in Winter. Bot. Gaz. 41 : pp. 373-424. 8 figs., 

1906. 



CHAPTER XVII 
THE LIGHT RELATION 

All green plants exhibit direct relations to intensities 
of light. The influence of light in the synthesis of the first 
organic products, or photosynthates, has been considered ; 
and it is now necessary merely to indicate some of the more 
general ecological relations. 

251. The adjustment of plant members. — No phenom- 
enon of plant life is more familiar than the turning of leafy 
shoots toward light or the orientation of leaves in a man- 
ner to occupy a favorable exposure. Plants placed at the 
window of a dark room promptfy show the effects of the 
light stimulus. The same relations may be observed in 
the field. The capacity to show through growth curva- 
tures an irritable response to light from one side is called 
phototropism. We have to distinguish as main classes 
of responding structures those axes which are parallelo- 
tropic, curving in such manner that the tips point toward 
or away from the source of light, and those which are 
plagiotropic, or at some angle. Leaves are transversely 
phototropic, and the response secures a favorable illumi- 
nation of the chlorophyll bodies. As a result broad- 
leaved plants develop commonly to form a more or less 
perfect mosaic, no better examples of which can be found 
than those of the grape-vine or Boston ivy. The adjust - 
115 



416 Plant Physiology 

ment of the single shoot, of the plant as a whole, or of a 
group of plants is of the same nature, ample considera- 
tion being given to other forces which may be operative 
at the same time. Trees thickly branched, such as the 
Norway maple and the linden, or the unpruned apple and 
pear, will exhibit in a significant manner this effect, ex- 
posing a complete shell of leaves. (See Chapter XX for 
growth movements.) 

When trees grow up close together in the forest the lower 
branches are ultimately too much shaded, so that these are 
killed and in time drop off. The leafy shoots are confined 
to the uppermost parts, and this system of constant self- 
pruning through the survival of those favorably placed 
results in the characteristic long trunks of the forest trees 
as compared with the shorter trunks and abundant 
branches of isolated specimens in the lawn or meadow. 
The tall trunks are, of course, most desirable from the 
standpoint of the lumberman; but, at the same time, the 
decayed branches or stumps offer favorable opportunity 
for the entrance of destructive fungi which cause great 
annual loss through the decay of sap or heart wood, and 
thus artificial pruning possesses great advantages. 

252. Light perception. — Phototropic organs may pos- 
sess special perception regions, and these regions do not 
necessarily correspond to those of curvature or bending. 
The method of perception is not understood, but the sensi- 
tiveness of the mechanism is almost incredible. 

The perceptive mechanism resulting in leaf orientation 
has received much attention. Haberlandt and others find 
in the lens-shaped cells and cuticular thickenings of epi- 
dermal cells the structures which they regard as indirectly 



The Light Relation 



417 



important. In these cells the light may be focused in some 
basal region of the pro- ^^__ 

toplasm, and through "Y" V"~~ "^"V""" jf^^"^? 
the unequal illumination a ^^^y^Yj = J^\ 
the stimulus to orienta- J | J J I ' I \ 

tion is supposed to be 
given. It has been dem- 
onstrated photograph- 
ically that these cells 
focus the rays, but since 
such cells occur under a 
variety of conditions, 
and for many other rea- 
sons, they are not posi- 
tively connected with 
this form of irritability. 
Important in the orientation is the 
sensitiveness of the petiole. Wager 




Fig. 113. Epidermal modifications which 
focus light rays ; Berberis (a) , Rhodo- 
dendron (6), and Primus Lauro-cerasus 
(c). [After Haberlandt.] 



direct or indirect 
believes " that the 
perception of light is bound up with its absorption by the 
chlorophyll grains, in which case the palisade cells would 
be the percipient cells." 

253. Diverse requirements. — From casual observa- 
tion of plant habitats, it may be noted that there is great 
diversity in the light intensity under which different species 
grow to maturity. Many plants reach perfection only 
when exposed, but others develop more vigorously under 
the partial shade of the forest or thicket. Exposed and 
shaded situations usually differ with respect to other en- 
vironmental factors, such as humidity and evaporation; 
and in a careful study of habitats it is necessary to measure 
and to attempt an evaluation of all factors. 
2e 



418 Plant Physiology 

It has been reported that during a unit interval the even- 
ing primrose utilizes in direct sunlight about three times 
as much C0 2 as when in diffuse light, while the common 
polypody works more effectively in the latter. Many 
shade-loving plants may reach maturity in light which 
is reduced to about 3V the intensity of maximum sunlight. 
Beyond a certain light intensity the plant gains little, for 
the small amount of C0 2 in the air is then the limiting fac- 
tor in growth. Temperature is a further limiting condi- 
tion. In the warm forest of the tropics there may be a 
vigorous forest-floor vegetation, but the cold shade of a 
far northern forest affords only a scant undergrowth. 

254. Light intensity. — Upon the surface of the earth 
light intensity varies considerably both diurnally and 
seasonally, depending, of course, with a clear sky. upon the 
altitude of the sun. The possible daily maximum is at 
sun-noon, June 22. If this intensity should be repre- 
sented at the equator by 100, then with a growing season 
in the north temperate zone approximately from March 
21 to September 23, or its equivalent in the southern 
hemisphere, the light intensity from 9 A.M. to 3 p.m. 
would be represented approximately by 82 to 98 and the 
noon variation by 93 to 98, as calculated by Clements for 
Lincoln, Neb. From this it is apparent that the variation 
in light intensity throughout agricultural regions with a 
clear sky during the growing season is not considerable. 
Range of intensity in the open is in general insufficient 
seriously to affect vigorous growth, although it may mod- 
ify form and chemical content. In some regions cloudi- 
ness may be an important factor. 

255. Injurious effects. — It has long been well estab- 



The Light Relation 419 

lished that injury may result from continuous or lengthy 
exposure to intense light. Many of the simpler green algae 
may be killed by an exposure to brilliant sunlight of less 




Fig. 114. Effect of light upon a plate culture of Pseudomonas campes- 
tris ; colonies have appeared only where the plate was protected by a 
letter (W) screen. [After Russell and Harding.] 

than one hour. It has been shown conclusively that strong 
light inhibits the action of various enzymes. The diastases 
are notably affected, so that the conversion of starch in a 



420 Plant Physiology 

clear solution may be readily prevented by exposure to 
light. The chlorophyll of shoots protects in a measure the 
diastase from the injurious action of light during the day. 
Nevertheless,, from this and other causes starch conversion 
in the leaf is reduced to a minimum during days of bright 
sunshine. 

Bacteria and other hyaline microscopic organisms are 
killed by direct sunlight, and this fact is important in sani- 
tation. The convincing demonstration of the effect of sun- 
light upon bacteria was made by Ward. He prepared 
cultures of the bacteria upon clear agar in Petri dishes, and 
then exposed the dishes to sunlight. It was determined 
that the organism causing anthrax, Bacillus anthracis, 
may be killed in such cultures in direct sunlight by an ex- 
posure of from a few minutes to several hours, depending 
upon the intensity of the light. Striking results were 
obtained by covering the dish to be exposed with a black 
paper or metal stencil so that the contrast between exposed 
and unexposed parts of the plate may be sharp. A spec- 
trum was also thrown upon prepared cultures and it was 
determined that the blue- violet rays constitute the effective 
killing portion of the spectrum. With the use of glass 
covers or globes the injurious rays are to a considerable 
extent excluded. 

256. Artificial light. — Interesting studies have been 
made upon the use of artificial light in greenhouse culture, 
as in forcing lettuce, endive, radish, and certain flowers. 
In such experiments the artificial light has been employed 
usually at night, or supplementary to daylight. Eco- 
nomically, artificial light is probably a failure, owing to the 
expensiveness of it ; but the results of the experimental 
work bring out some points of interest. 



The Light Relation 



421 



By the use of the protected electric arc during half of the 
night Bailey was able to hasten lettuce two weeks. The 




Fig. 115. Lettuce of the same age under normal sunlight (above) and 
with electric arc a part of the night in addition (below). [After Bailey.] 

naked electric arc yields light distinctly injurious to the 
majority of crops. This injury is due to the richness in 
ultraviolet rays, which, as already shown, are destructive 
to protoplasm. When screened by glass, clear or opales- 



422 



Plant Physiology 



cent, the harmful rays are largely excluded. Continuous 
night illumination with the electric arc may promote more 

rapid growth in some 




but 



ith others 
a tendency to 



plant 
there is 
run to seed. 

The incandescent elec- 
tric light, which is rela- 
tively rich in red rays, 
has been successfully em- 
ployed by Rane in forc- 
ing lettuce. By the use 
of the acetylene light 
Craig has found it possible 
to force the growth of 
radish, lettuce, and a few 
other crops ; but the best 
results were with flowers, 
Easter lilies especially giv- 
ing increased production 
in a shorter time. 

257. Monochromatic 
. , light. — Many experi- 

Fig. 116. Field peas grown for equal ° x 

periods in white (a), blue (6), and ments have been made to 
orange-yellow (c), light. determine approximately 

the effects of light of different wave lengths on the form 
and structure of plants. In much of the work which 
has been done pure screens were not employed, yet his 
type of work is sufficiently important to justify careful 
physical methods. In general, the dry weight of plants 
grown for a considerable period under monochromatic 



The Light Relation 



423 



screens is greatest in red, and least in violet ; yet the 
growth in red is not equal to that in white light. The vio- 
let rays are also important in the production of bloom. 

In the following table there are given, after Teodoresco, 
the relative areas of leaves developed from the bud in dif- 
ferent qualities of light during a period of about thirty 
days. With each plant the leaves occupied equivalent 
positions on the young shoot : — 

Effect of Wave Length upon Area of Leaves, Areas in sq. mm. 







Kind of Light 














White 


Red 


Green 


Blue 


Darkness 


Vicia Faba 


948.3 




127.8 


654.8 


52.8 


Lupinus albus 


158 


49.5 


38 


62 


8 


Polygonum Fagopyrum (coty- 












ledons) • 


128 


59 


23 


64 


11 


Ricinus sanguineus (cotyledons) 


1105 


503 


200 


600 


53 



The next table indicates the thickness of the leaf under the 
different conditions of illumination, and also the number of 
stomata on equal areas of the lower surface : — 





White 


Red 


Green 


Blue 


Plant Employed 


Th. 
Leaf 


No. 

Stom. 


Th. 

Leaf 


No. 

Stom. 


Th. 
Leaf 


No. 

Stom. 


Th. 
Leaf 


No. 
Stom. 


Vicia Faba . . . 
Arachis hypogaea . 
Ricinus sanguineus 
Lupinus albus . 


671 
243 
346 
352 


4 
12 

2 
14 


337 
216 
256 
210 


7 
18 

6 
31 


297 
207 
218 
195 


12 
20 
10 

38 


332 
216 
297 
271 


7 
13 

6 
25 



424 



Plant Physiology 



258. Half-shade in plant propagation. — In conserva- 
tory and greenhouse production of certain tropical plants 
whose habitats are naturally the moist, shady woods, some 
form of shading has commonly been practiced. It is only 
in this manner that many delicate ferns and succulent 
species are grown successfully. The same purpose is ef- 
fected by the cloth-covered tents or slat-covered sheds 







Fig. 117. A coffee plantation in a Hawaiian forest. [After Van Leen- 
hoff.] 

often employed in southern climates, ostensibly to dimin- 
ish the light, but also to insure higher humidity and to pro- 
tect against wind and frost. As a result of such work it 
has become apparent that partial shade may be advanta- 
geous in many horticultural or even farm crop operations. 
The demand for vegetable products out of season is an ad- 



The Light Relation 



425 



ditional incentive to the 
use of any device which 
may regulate or control 
the conditions of the en- 
vironment. 

259. Crops responding 
to half-shade. — In gen- 
eral half-shade increases 
succulence and delicacy, 
so that it is particularly 
applicable to such crops 
as asparagus, cauliflower, 
celery, lettuce, and rad- 
ish. It is employed in 
forcing rhubarb, in the 
cultivation of ginseng, 
and has proved especially 
important in pineapple 
culture in Florida. 
Sumatra tobacco, grown 
for wrapper purposes in 
the Connecticut valley 
and in other regions, has 
been greatly improved by 
half-shade conditions. 
Half-shade is also neces- 
sary in many cases to 
the maintenance of 
proper conditions for seed 
beds, and it is essential in FlG - 11 / 8 ; Pe f s r ™ n J d ^ s ^ dark- 

ness (a), in about Hignt (o), and open 

the nursery propagation i n greenhouse (c). 




426 Plant Physiology 

of many forest trees. A thorough study of the relations 
of seedling trees to half-shade operations is greatly to be 
desired in the advancement of forestry work generally. 

Many bush fruits and other agricultural products are 
commonly grown in the partial shade of other plants. It 
is generally believed that currants are benefited by the 
partial shade of grapes or certain tree-fruits, provided the 
water-content of the soil is not seriously affected. Coffee 
and tea are more profitably produced in subtropical regions 
in forest glades, or when partially shielded by occasional 
trees. In southern Algeria and other portions of the Sa- 
hara shading is practiced on a large scale in the oasis cul- 
tures. The protecting palms improve the conditions for 
figs, peaches, and other fruits, under which, in turn, vege- 
tables may be grown, provided only that the water-supply 
is adequate. On the other hand, the production of grapes, 
with higher sugar content for wine purposes may require a 
selection of slope insuring best exposure to light during a 
certain period of growth and maturity. Fruit trees are 
grown on the southern sides of walls in England and 
France, and it would appear that both the additional light 
and heat thus obtained are advantageous. 

In many parts of the United States, especially in the 
Central West, lettuce and other salad crops become bitter 
and undesirable for table use with the stronger light and 
heat of the summer season. The shade tent, properly 
employed, will permit the constant summer culture of 
such crops. The strong flavor of radishes produced dur- 
ing the summer are also modified and improved by partial 
shade. 

260. Morphogenic effects. — The comparative effects 



The Light Relation 



427 



of light and darkness upon the form and growth of plants 
has been much investigated. In most cases no adequate 




Fig. 119. Sun print showing difference in opacity (thickness and chlo- 
rophyll content) of celery leaves grown in half -shade (left) and in sun- 
light (right). 

consideration has been given to other factors than light, 
but in general the response to light is so much more marked 



428 



Plant Physiology 



than to other factors that the errors are perhaps negligible. 
When the buds of ordinary herbaceous plants with central 
axes are permitted to develop in the dark there is a marked 
elongation of the internodes and a suppression of branches. 
Shoots from tubers or tuberous roots affording constant 
food-supply will show this characteristic in a striking man- 
ner. The leaves of such herbaceous plants are usually 








Fig. 120. Sumatra tobacco under cloth 
[After Shamel.J 



Connecticut Valley. 



greatly reduced in size, and sometimes restricted to mere 
scalelike structures. On the other hand, when the light in- 
tensity is reduced to from 20 to 40 per cent of normal sun- 
light, the leaves may be increased to twice their size in di- 
rect sunlight, as demonstrated by many experiments upon 
lettuce, tobacco, and other broad-leaved plants. When 
produced in the dark, the radicle leaves of such plants as 
the rhubarb develop in a short time petioles of unusual 
extent and delicacy, while the leaf blade remains small. 



The Light Relation 429 

This effect is of much practical value in the forcing of rhu- 
barb for early market (Fig. 87). 

The leaves of tobacco produced in the shade tent are not 
only larger, but thinner, and they possess relatively larger 
air spaces and more spongy parenchyma ; the fibrovascu- 
lar bundles, or venation systems, are less prominent, and 
the leaf is thereby improved for wrapper purposes. In the 
fibrovascular bundles of half-shade plants the mechanical 
supporting tissues are usually reduced, and this is a factor 
in blanched celery. Self-shading to produce crispness and 
tenderness may be practiced in some cases ; thus in the 
cultivation of Cose lettuce, romaine, and cauliflower, 
the simple operation of bringing together and tying the 
leaves in the form of a head may produce the effect 
desired. 

261. Half-shade and quality. — Plants grown in half- 
shade commonly contain a higher per cent of moisture and 
less ash. It has been ascertained that the apparent acid- 
ity of strawberries is increased by shade. This apparent 
increase is, however, due to lessened accumulation of 
sugar in the berries. 

The aromatic products of plants are not important as 
animal nutrients, but they are physiologically essential, 
and represent almost the sole value of many economic 
plants used as condiments. In 1838, De Candolle called 
attention to the diminished production of savors and odors 
in shaded plants. It was found later that plants removed 
from southern latitudes to the latitude of Scandinavia 
during the two months of maximum sunshine in the latter 
region, showed an increase in the development of aromatic 
products. Indeed, it has long been suggested that many 



430 



Plant Physiology 



fruit-bearing plants containing objectionable flavors might 
be improved by reduced light. 1 

262. The effect of shading upon other environmental 




Fig. 121. Bark of Acer Pseudo-platanus ; epidermis (ep), periderm (pr), 
primary cortex (pc) ; developed in white light (A) and in red light (B). 
[After Teodoresco.] 

factors. — From the preceding statements it has been noted 
that half-shade may modify in a direct manner other condi- 
tions of the environment. The factors commonly affected 
are the following: (1) moisture conditions of the soil; 
(2) rate of evaporation ; (3) humidity ; (4) temperature ; 
(5) air movement; and (6) certain biological relations. 

The tables on the next page indicate the effect of the 
usual shade tent (made of unbleached cotton) upon soil 
moisture and evaporation (first table after Whitney). 

Much remains to be determined respecting the modifica- 
tions in plants induced by shading, and likewise exten- 
sive studies are required to evaluate the different factors 
involved. From the indications already presented, it is 



1 Schuebeler, Bonnier, and Flahault have shown that in northern 
climates flowers are more highly colored and plants commonly richer in 
essential oils. It is also well known that plants rich in volatile oils and 
other aromatic products are numerous in the Mediterranean region, a 
region in which the rainy season is confined largely to the winter months, 
and the summer is practically a continuous exposure to intense sunlight 



The Light Relation 



431 



obvious that many of the effects conveniently discussed 
under " shading," in the sense of " half-shade," are in 
large part the results of changes in the water or mois- 
ture relations. The relation of shade plants to fungous 
diseases also deserves a more careful study. 

Table of Soil Moisture Inside and Outside of Shade 
(Cloth) Tent 



Inside Outside 



Inside Outside 



July 1 

July 2 

July 3 

July 4 

July 5 

July 6 

July 7 

July 8 

July 9 

July 10 



15.2 
15.0 
15.2 
13.7 
12.8 
12.1 
12.0 
12.2 
11.5 
11.0 



12.3 
13.3 
13.4 
11.5 
9.9 
9.7 
8.4 
7.8 
6.9 
6.3 



15.6 
15.4 
15.3 
14.9 
13.5 
13.9 
14.0 
13.1 
13.0 
12.7 



13.6 
14.0 
14.3 
13.4 
12.1 
11.8 
10.8 
10.5 
9.8 
8.7 



EVAPORIMETER READINGS, SHADE-TENT EXPERIMENTS, 

Ithaca, N.Y., 1908 



Cc. of Water Lost by 
Standardized Evaporimeters 



Closed Tent Open 
Tent North 



Aug. 11-23 . . 
Aug. 23 — Sept. 1 
Sept. 1-8 .. . 
Sept. 8-15 . . . 



85 
80 
75 



163 
162 
140 
110 



263 

240 
210 
170 



432 Plant Physiology 

LABORATORY WORK 

Orientation. — Make and record observations in the open or 
in the greenhouse upon the relations of shoots and leaves of 
any plant to light. Begonia, grape, or Norway maple may be 
used ; also note the relations of the compass plant (Lactuca 
Scariola) if available. 

Place a pot or water culture containing seedlings (several 
centimeters high) in a chamber permitting one-sided illumina- 
tion. The chamber may consist of a tight box, black on the 
inside, arranged with a slit on one side through which rays of 
light may be admitted. Place the plants as far as possible 
from the source of light, and for some hours note the response 
of the shoot (and also of the root if a water culture is employed). 
Expose another plant which has been in complete darkness to 
one-sided illumination for some moments and then return it to 
a dark chamber. Note any subsequent response and discuss 
the results. 

Light perception. — Make hand sections of leaves of oats, 
hyacinth, hepatica, Saxifraga Geum, or Garrya elliptica, and 
describe the lens-shaped cells or epidermal modifications con- 
sidered by Haberlandt and some others to be light-perceptive 
organs. Consult the article cited by Wager, note his method 
of photographing objects through cells, and read his conclusions 
regarding light perception. 

Wave length and rate of growth. — With bottles, test-tubes, 
and corks prepare three pieces of apparatus as shown in Fig. 
122. Prepare the solutions of (1) ammoniacal copper carbonate 
and (2) naphthol yellow, so as to give practically pure colored 
lights (spectroscopically tested, if possible), the one excluding 
practically all except blue and blue violet rays, and the other 
excluding all except the red end of the spectrum. Fill one 
bottle three fourths full with each of these solutions and one 
with water. Place in each test-tube, on filter paper or moss, a 
germinated seed of the field pea, and insert the tubes as shown 
in the figure. Relative growth may be observed until the seed 
have outgrown the chambers. With the apparatus commonly 



The Light Relation 



433 



at hand, it is impracticable to at- 
tempt to compute equal energy inten- 
sities of the colored lights. 

Light intensity. — Determine the 
relative value of light in the open 
and contrast it with the light inten- 
sity in the greenhouse and in the 
shade of vegetation or buildings. To 
make this study use the ordinary 
photographic actinometer, the device 
employed by Clements, 1 or strips of 
solio paper. If the latter are em- 
ployed, it is simplest to determine the 
length of exposure in seconds neces- 
sary to bring the paper to a certain 
standard shade of brown. This may 
be done by previously following the 
changes in the paper while contrast- 
ing it with a brown color scheme, 
choosing some shade of color in the 
color scheme as a standard which is 
invariably one of those attained by 
the paper in the process of darkening. 

Etiolation. — Place in a perfectly 
dark chamber water cultures of peas, 
potatoes, and onions sprouting on 
moist moss, and any potted plants 
available. Make accurate observa- 
tions of the conditions of the plants 
or buds when placed in the dark, and, 
if possible, arrange control cultures 
exposed to the light, but under sim- 
ilar conditions of moisture and tem- 
perature. After ten days or more, 
make comparative observations, not- 
ing the effect (1) upon structures 

1 Physiology and Ecology, pp. 72-75. 
2f 





Fig. 122. Simple apparatus 
for qualitative tests of the 
effects of light of different 
wave length. 



434 



Plant Physiology 



developed in the dark ; (2) upon structures formed previous to 
placing the plants in the dark. 

Secure the same variety of any plant grown in half-shade and 

in an exposed situation ; 
compare the two with re- 
spect to structural modifi- 
cations, water-content, and 
extent of root system. 

Light and blossoms. — 
Place over carnations, just 
coming into blossom, aer- 
ated bell glasses, one of the 
bell glasses being covered 
with manila paper or un- 
bleached cotton. Follow 
the effect of severe shading 
upon the opening of flower- 
buds of other plants which 
were equally advanced at 
the outs.t . 

Killing effect of light. — 
Fig. 123. Potato sprouting in a dark, Prepare in a Petri dish a 
moist atmosphere. dilution culture of any 

species of bacteria convenient, using the minimum quantity of 
the clearest agar obtainable. When the agar is solidified, expose 
the cultures about one hour to direct sunlight, protecting, how- 
ever, a portion of the dish by means of darkened cardboard. 
Replace the cover of the dish, incubate the cultures for several 
days, and note the effect of the exposure to light. This experi- 
ment cannot be carried out where laboratories are not equipped 
for the cultivation of micro-organisms. 




References 



Bailey, L. H. Some Preliminary Studies of the Influence of the 
Electric Arc Lamp upon Greenhouse Plants. Cornell Agl. 



The Light Relation 435 

Exp. Sta. Bui. 30 : 83-122, 1891 ; 42 : 131-146, 1892 ; 55 : 
145-157, 1893. 

Duggar, B. M. Shading Plants. Cyclopedia of American Ag- 
riculture. 1 : 119-123, 1907. 

Haberlandt, G. Lichtsinnesorgane der Laubblatter, 1905. 

Kniep, H., und Minder, F. Ueber den Einfluss versch. Lichtes 
a. d. Kohlensaure-assimilation. Zeit. Bot. 1 : 619-650, 1909. 

MacDougal, D. T. The Influence of Light and Darkness upon 
Growth and Development. Memoirs N.Y. Bot. Gard. 2 : 
319 pp., 176 figs., 1903. 

Rane, F. W. Electro-Horticulture. W. Va. Agl. Exp. Sta. 
Bui. 37 : 1894. 

Siemens, C. W. On the Influence of Electric Light on Vegeta- 
tion. Proc. Roy. Soc. 30:210-219. 

Stewart, J. B. The Production of Cigar- Wrapper Tobacco 
under Shade in the Connecticut Valley. Bur. of Plant Ind. 
U. S. Dept. Agl. Bui. 138 : 31 pp., 5 pis., 1908. 

Stone, G. E. Response of Plants to Artificial Lights. Cyclo- 
pedia of American Agriculture. 2 : 22-27, 1907. 

Teodoresco, E. Influence des differentes radiations lumi- 
neuses sur la forme et la structure des plantes. Ann. d. Sci. 
Nat. (Bot.). 10 (ser. 8) : 141-263, pis. 5-8, 1899. 

Wager, H. The Perception of Light in Plants. Ann. Bot. 
23 : 459-489, 2 pis., 1909. 

Whitney, M. Growing Sumatra Tobacco under Shade. Bu- 
reau Soils, U. S. Dept, Agl. Bui. 20:31 pp., 7 pis., 1902. 

Texts. Clements, Jost, MacDougal, Pfeffer. 



CHAPTER XVIII 

RELATION TO DELETERIOUS CHEMICAL 
AGENTS ' 

A large number of water-soluble chemical substances 
are injurious to all living protoplasm at concentrations 
considerably below the osmotic equivalent of the cell-sap. 
Such injurious substances are poisons, or toxic agents. 
These may act directly or indirectly upon the protoplasm, 
and the inference is that the action is ultimately chemical. 
The dilution of a deleterious agent often results in stimu- 
lation, whilst at still further dilution this effect also 
disappears. 

There is at present very incomplete knowledge of toxic 
action; yet many advances have been made within the 
past quarter-century. These advances have served to in- 
crease knowledge generally, and in agricultural lines they 
have been important in the study of soils, bacteriology, 
plant patholog} r , and entomology. The results have been 
utilized in the interpretation of experiments with fertiliz- 
ers, in improving methods of disinfection or purification of 
water-supplies, in the protection of plants against insect 
pests and fungous diseases, and in various other ways to 
which subsequently subsidiary reference may be made. 

263. General relations to poisons. — - Toxic agents may 
be general or specific poisons. Specific poisons are as yet 
436 



Relation to Deleterious Chemical Agents 437 

of minor importance in plant work. General poisons are 
usually either strong (such as salts of mercury), or weak 
(alcohol) for all organisms. Nevertheless, plants may 
show some specific adjustment to poisons, and diversity 
in effect may be due to one of the following causes : — 

(1) A certain selective absorption may be shown, as in 
the case of the nutrients, so that penetration will be rapid 
in one case and practically prevented in another. 

(2) Upon penetration the deleterious substance may be 
"converted into a relatively insoluble and nontoxic form, 

before effecting serious injury to the protoplasmic organi- 
zation. 

(3) There may be specific differences in the effects upon 
protoplasm, — peculiarities which it is at present impos- 
sible to explain definitely. 

One parasitic fungus may be killed by a dilute solution 
of a copper compound, and another may germinate in a 
relatively concentrated fungicide. Again, alkaloidal or 
other toxic organic bodies may be produced within living 
tissues, where they seem to set up no particular disturb- 
ance ; whereas they may serve as strong toxic agents 
when placed in contact with other cells or organisms. In 
the fermentation of fruit sugar the common yeast plant 
produces alcohol, which soon prevents the growth of other 
micro-organisms. Brown has demonstrated a marked 
selective permeability in the coverings of seeds of a variety 
of barley. These seeds take up water from a fairly strong 
solution of sulphuric acid, and remain uninjured; but 
mercuric bichlorid penetrates them with comparative ease. 

264. Comparative resistance. — The fungi and bacteria 
are commonly much more resistant to toxic agents than are 



438 



Plant Physiology 



species of seed-plants ; that is to say, many fungi and bac- 
teria may grow in solutions which would inhibit root 
growth. However, aerial surfaces of seed-plants do not, 
as a rule, permit the rapid absorption of water or of chemi- 
cal agents, so that for crop protection such surfaces may 
be covered with strengths of toxic solutions prohibitive 
to the germination and growth of fungi, as in the use of 
fungicides and insecticides. 




160 grams 
sand 



200 grama 

sand 



Fig. 124. Depression of toxicity with addition of sand. [After True 
and Oglevee.] 



Relation to Deleterious Chemical Agents 



439 




440 Plant Physiology 

Many species of the lower algae are particularly sensi- 
tive to certain toxic agents, such as the salts of copper and 
other heavy metals. Zoospores of fungi and some species 
of bacteria pathogenic in animals may be equally sensitive. 

265. Toxic action and the substratum. — Much of the 
literature of toxic action is confusing, owing to the fact that 
the results are not comparable. Substances usually ex- 
hibit their greatest toxicity in distilled water. Any nearly 
neutral nutrient solution reduces toxic action even in cases 
where molecular readjustments would not seem to be im- 
portant. In the soil complex physical and chemical con- 
ditions prevail, and these further modify toxic action. 

Solid particles, such as pure sand, graphite, and filter 
paper, may reduce toxic action to a considerable extent. 
True and Oglevee found that twice as much sand as solu- 
tion may reduce the toxic action of CuSCh for Lupinus 
albas as much as thirty-two times (Fig. 124). The method 
of reducing toxicity by solid particles is usually denoted ad- 
sorption. It is a phenomenon explained upon the hypoth- 
esis that many molecules or ions of the toxic substance are 
physically held by the surfaces of the particles of the inert 
material, and are, for the time, removed from the possi- 
bility of chemical action. Another explanation is that the 
solid substances offer obstacles to the free movement of 
the solvent particles. Possibly both views are important. 
Many of the so-called absorptive properties of soils both 
respecting fertilizers and deleterious agents are in reality 
adsorptive. 1 

1 The table from Jensen, on the opposite page, affords a comparison of 
toxic action in sand and in solution cultures. 



Relation to Deleterious Chemical Agents 441 

From the data presented, it is evident that in denning 
toxic concentrations it is necessarj^ to speak in terms of the 
substratum. When soil cultures are employed, the type 
of soil and amount of organic matter are important. The 
nutrient solution may modify the action of a poison by 
forming with it chemical combinations less diffusible or 
dissociated, and ultimately less injurious. Again, there 
may be antitoxic action, as in the calcium-magnesium 
relation. Mass action is also important, as suggested by 
Dandeno ; thus a seedling injured by 5 cc. of a toxic agent 
may be killed by a greater quantity of the same concentra- 
tion. 

266. Method of action. — It is not possible at present 
to state definitely the method of action of all deleterious 
agents. Many metallic salts and other substances pre- 
cipitate protein, and it is easy to picture the immediate 
disturbance of protoplasmic organization effected by such 





Soil Culture (Quartz Flour 
with Nutrients) 


Solution Culture 
(with Nutrients) 


Toxic 
Agent 


Parts of an N/10,0 


00,000 Solution 




Inhibiting 
Growth 


Stimulating 
Growth 


Inhibiting 
Growth 


Stimulating 
Growth 


Ni(N0 3 ) 2 


70,000- 60,000 


5,000- 1,000 


5,000- 2,500 


4-2 


ZnS0 4 . . 


300,000- 100,000 


3,000- 1,000 


7,000- 6,000 


none 


AgN0 3 . 


300,000- 100,000 


90,000- 10,000 


1,000- 900 


20-10 


CuS0 4 . . 


300,000- 100,000 


10,000- 4,000 


10,000- 5,000 


none 


Fe 2 Cl 6 . . 


600,000- 400,000 


90,000- 20,000 


100,000- 8,0000 


4,000- 2,000 


Pb(N0 3 ) 2 


500,000- 300,000 


90,000- 40,000 


400,000- 200,000 


20,000-10,000 


Phenol . 


200,000- 100,000 


8,000- 4,000 


200,000- 100,000 


8,000- 4,000 


Alcohol . 


7,500,000-2,500,000 


750,000-250,000 


7,500,000-2,500,000 


75,000-25,000 



442 



Plant Physiology 



agents, yet it is impracticable to adopt a special grouping 
based upon a similarity of action within the cell. Among 
the deleterious agents known, those of economic signifi- 
cance are of special interest. Of these the important groups 
are inorganic and organic acids ; caustic alkalies ; salts of 
the heavy metals ; formalin ; alcohol and anaesthetics ; 




Fig. 120. Indications of the effects of the substratum upon the toxic 

action of CuS0 4 ; loam (L), sand (S), graphite (G). [Photograph by 
W. W. Bonus.] 

various organic compounds, including decomposition and 
hydration products of proteins and lecithins, alkaloids and 
miscellaneous nitrogenous bodies, also many non-nitroge- 
nous organic products of diverse composition ; and cer- 
tain deleterious gases of the carbon series. 

267. Inorganic and organic acids. — Inorganic acids are 
usually the most toxic of the acid substances for the higher 



Relation to Deleterious Chemical Agents 443 

plants. Some of the results secured by Kahlenberg and 
True are given in the table below, where also a compari- 
son may be made with acetic acid, the latter occupying 
an intermediate position with respect to toxicity among 
organic acids : — 





PlSUM SATIVUM 


Zea Mays 


LUPINUS ALBUS 


Acids 
















Gram Mol. 


Parts Per 


Gram Mol. 


Parts Per 


Gram Mol. 


Parts per 




Sol. 


Million i 


Sol. 


Million 


Sol. 


Million 


HCL . . . 


1/12800 


3 


1/3200 


11 


1/6400 


5.5 


H 2 SO* . . . 


1/12800 


3 


1/3200 


11 


1/6400 


5.5 


HN0 3 . . . 


1/12800 


3 


1/3200 


11 


1/6400 


5.5 


CH3COOH . 


1/3200 


11 


1/400 


91 


1/1600 


22.5 



In these experiments the roots of a few seedlings were 
immersed in 300 cc. of solution (the acid in distilled water) 
and the concentrations given are just sufficient to kill at 
least 50 per cent of the roots after an exposure of 24 hours. 
The toxicity of inorganic acids is strikingly reduced by the 
presence in the solution of solid particles. 

268. Alkalies. — Alkalies are in general less toxic to the 
roots of seed plants than are equivalent concentrations of 
acids or of salts of the heavy metals. In order to inhibit 
root growth of seedlings in water cultures, it requires from 
5 to 10 times as strong a solution of caustic alkali as of a 
mineral acid. Alkalinity (basicity) and acidity as applied 
to field conditions are merely relative terms, since under 
such conditions the usual methods of determining these 
qualities are inaccurate. It is well known, however, as in- 

1 Approximate. 



444 



Plant Physiology 




446 



Plant Physiology 



dicated under the discussion of lime, that many plants 
thrive under basic conditions, while others yield best when 
the substratum is acid. 

269. Salts of the heavy metals. — The salts of the 
heavy metals constitute a group of the most toxic agents 
known. The various soluble inorganic salts of the same 
metal are commonly of about equal toxic value. The 
table given below is comparable to that given for inorganic 
acids, the concentrations representing those which kill the 
majority of the roots in 24 hours : — • 



1'isr.M sativum Zea Mats Lupinus albus 



Gram Molecular Solution 



CuCl 2 . 
CuS0 4 . 

NiS0 4 . 
Ni(N0 3 ) 2 
CoS0 4 . 
Co(N0 3 ) 2 

AgNO:, . 

Ag 2 S0 4 . 
HgCl 2 . 
KCN . 



1/51200 

1/51200 

1/51200 

1/51200 

1/25600 

1/25600 

1 204800 

1 204800 

1/204800 

1/12800 



1/102400 

1/102400 

1/51200 

1/51200 

1/6400 

1/6400 

1 204800 

l 204800 

1/51200 

1/6400 



1/25600 

1/25600 

1/25600 

1/25600 

1/12800 

1/12800 

1/204800 

1/204800 

1/12800 

1/6400 



Kanda employed pots holding two liters of soils in some 
experiments with horse beans. These were in one case 
watered daily with copper sulfate to such extent that at the 
end of three weeks the pot contained 26.394 grams of the 
salt. This amount caused only a slight reduction of root 
growth, but stem growth was greater than in the control. 

Copper compounds are extremely injurious to certain 



Relation to Deleterious Chemical Agents 447 

algse. They have been effectively employed by Moore 
and Kellerman l for the eradication of such organisms in 
ponds and water supplies. For this purpose copper sul- 
fate is used at the rate of 1 part to 250,000-1,000,000 
parts of water. A copper coin in a small dish of water 
containing half a dozen threads of a green alga is sufficient 
to cause death in a day or two. 

270. Formalin. — Formalin is a penetrating toxic agent 
for all plant cells. According to Clark it ranks close to 
mercuric bichlorid and silver nitrate as a poison for fungi 
in beet decoction. In agricultural practice formalin so- 
lutions are important in the control of certain fungous 
diseases by seed treatment. The seed do not absorb the 
solution so rapidly as the spores, so that a short immersion 
may serve to disinfect the former. Formalin is employed 
for the prevention of bunt of wheat, loose smut of oats, 
and potato scab. 

271. Organic bodies. — The effects of various alkaloids 
and other nitrogenous bodies upon the higher vertebrates 
have long been a matter of experimentation. The toxic 
products of disease-producing bacteria are of this nature. 
Such substances are frequently more toxic to organisms 
possessing complex nervous and circulatory systems ; but 
similar substances may be injurious to protoplasm in gen- 
eral. Through the decomposition of animal or vegetable 
matter in the soil, toxic bodies may be formed, and these 
may at times play a recognizable role in the relations of 
vegetation. 

272. Root excretions. — De Candolle made the sugges- 
tion more than half a century ago that plants may influence 

1 Bureau Plant Ind., U. S. Dept. Agl., Bui. 64 : 44 pp., 1904. 



448 Plant Physiology 

one another by means of substances derived from their 
roots. This view was at first credited, but soon lost sup- 
port. Rotation of crops is based largely upon the idea of 
physical advantage, or disease suppression. In very 
recent years some investigators have proposed that soils are 
commonly unproductive on account of the presence in them 
of toxic organic compounds. This view with some persons 
implies that the injurious substances arise through the ex- 
cretions of roots. The assumption of any general excre- 
tion of toxic bodies by roots is at present scarcely justified, 
although an oxidizing power of roots is now demonstrated. 
273. Unproductiveness. — Interesting and valuable 
data have been accumulated by Schreiner and his asso- 
ciates, which throw much light upon the nature of the or- 
ganic compounds which may be found in the soil, and like- 
wise upon their toxicity. The decomposition of root-hairs 
and cast-off portions of roots, of green manures, or of any 
plant or animal remains in the soil give rise to temporary 
products which may be injurious. Nevertheless, it is not 
believed that the quantities of injurious organic bodies 
set free in a well cultivated soil during the growth of a 
staple crop, whether due to the decomposition of roots or 
to direct excretion, are often sufficient to be of agricultural 
importance. With the large number of bacteria ordinarily 
present in the soil, and the amount of aeration necessarily 
given in cultivation, such toxic substances would seem to 
be of merely temporary concern. In the case of bog soils, 
or land where there is insufficient drainage and lack of 
aeration, the toxic factor may be permanently important. 
It is certain that unproductiveness is not due to a single 
factor of this type, and at present many lines of work are 



Relation to Deleterious Chemical Agents 449 

being directed toward a solution of the problems of in- 
fertility. 

274. Relative toxicity of some organic compounds. — 

In the table below are given some of the interesting results 
obtained by Schreiner and Reed respecting the effects of 
various organic compounds upon wheat placed in water 
cultures from 7 to 10 days ; the concentrations indicated 
are in parts per million (p. p.m.) in distilled water : — 



Substances 


Lowest 
Conc. 

CAUSING 

Death, 
p.p.m. 


Lowest 

CONC. 

CAUSING 

Injury, 
p.p.m. 




Alanine 
Tyrosine . 
Leucine 
Choline 








500 
10 

500 


Only roots injured at 500. 

No injurious action. 
Roots most affected. 


Neurine . 






250 


25 




Betaine 
Guanine . 
Guanidine 






100 


1 


No injury. 
No injury. 


Skatol . . 
Pyridine . 






200 


50 
50 


Roots more injured. 



Among twenty-two nitrogen-containing compounds, two 
were found which were injurious at the surprisingly low 
concentration of less than 10 p.p.m. In the above ex- 
periments water cultures were employed ; but tests by 
Bonns in my laboratory have shown that wheat in paraf- 
fined pots containing rich garden loam is practically un- 
affected by pyridine at the enormous rate of 8000 p.p.m., 
this solution being used to moisten the soil to 60 per cent 
of its water-holding capacity. If this relationship should 
2g 



450 Plant Physiology 

be found to hold good for the other organic substances 
mentioned, it is apparent that the accumulation of such 
bodies in the soil in amounts which might be toxic (wholly 
neglecting the possibility of their immediate destruction) 
would require long periods of irrational cropping. 

275. Illuminating gas. — It has long been known that 
illuminating gas is injurious to vegetation. Even small 
leaks in gas pipes are fatal to the roots of trees in the vicin- 
ity. Vegetation in cities suffers greatly from this cause . 
The danger is greatly increased by the fact that gas diffuses 
through the soil to considerable distances, particularly 
when the surface of the ground is frozen or compact, as 
when streets or roadways supervene. Many decorative 
plants are reported to fail as house plants when illuminat- 
ing gas is burned. This may be due to gas-escape at the 
time of lighting burners (since, as will be shown subse- 
quently, the amount of gas needed to cause injury is ex- 
tremely small), or it may be due to incomplete combustion 
of the gas. 

Crocker and Knight have shown that ethylene, although 
present in very minute quantities, is apparently the chief 
toxic constituent of the illuminating gas with which they 
worked. They employed as indicators flowers and buds 
of the Carnation, — Boston Market and the pink Lawson. 
After an exposure of three days the young buds of these 
plants were dead, and bursting buds were prevented from 
opening by a concentration of one part of gas in 40,000 
parts of air ; while after an exposure of twelve hours 1 part 
to 80,000 caused the flowers that were already opened to 
close. In ethylene of 1 part in 1,000,000 buds in which 
the petals were just showing failed to open after an ex- 



Relation to Deleterious Chemical Agents 451 



posure of three days, and flowers closed after an exposure 
of twelve hours to an atmosphere of only 1 part to 2,000,000. 
276. Stimulation by means of weak toxic agents. — Small 
quantities of acids and other substances may serve as 
stimulants in several types of enzyme action, — they may 
increase the velocities of the chemical reactions. The 
transformation of starch by diastase and of certain pro- 
teins by pepsin are both accelerated by traces of acid. 
Richards and Ono have shown conclusively that the dry 
weight of certain fungi in nutrient solutions may be in- 
creased two or three times by the addition of a small 
quantity of one of several metallic salts. 1 In general, 
zinc has afforded the best results. Spore production is 
diminished in the stimulated cultures. Furthermore, it 
has been shown by subsequent work that stimulated plants 
are able " to dispose more economically of the sugar 
used, . . . thereby permitting a more rapid production of 
dry substance in a given time." 

1 The data from some experiments (Richards) in which the fungus was 
grown on a nutrient solution containing sugar are as follows : — 

Stimulation of Growth in Aspergillus niger by ZnS04 
(Cultures kept at 30° C. unless otherwise Noted.) 



Special Conditions 
of Culture 


Period 

of 

Growth, 

Days 


Weight in Milligrams 


No 
ZnS0 4 


.002 % 
ZnS0 4 


.004% 
ZnS0 4 


.008 % 
ZnS0 4 


.016% 
ZnS0 4 


.033 % 
ZnS0 4 


Asparagin added . 
Peptone added 
2%FeS0 4 added. 
24° C 


7 
6 
6 
8 
6 
6 


335 
300 
560 
1120 
650 
200 


730 
670 
970 
1490 
630 
335 


760 
700 
980 
1375 
680 
350 


765 
680 
770 
1515 
650 


770 
650 
390 
1455 
645 


715 
610 
250 
1280 
610 



452 Plant Physiology 

The results of stimulation experiments in which seed- 
plants have been employed are somewhat contradictory. 
On the whole, a certain degree of stimulation seems pos- 
sible, especially when some of the conditions of growth are 
unfavorable. From investigations conducted in Japan 



Mgm. 
dry wt. 



300 
200 






::|^1---|:' ! "1 : -"^ -\ ■ •-i-';-^ - \^ : :-\^^\ 



J 



Control .0005 .001 .002 .0085 .007 .015% ZnS0 4 

Control .002 .004 .008 .016 .088---- %NaFl 

Fig. 129. Stimulation of growth in Aspergillus by ZnS0 4 (continuous 
line) and NaFl (broken line). [Data from Richards.] 

there have been reported some benefits from the joint 
action of iron and manganese, also with salts of iodin, so- 
dium fluorid, and some other agents, yet some of the effects 
are probably indirect. 

277. Protection of crops by insecticides and fungi- 
cides. — Practically all cultivated or exploited crops are 
subject to the attacks of insects and fungi. More than 
twenty-five important species of insects are known to at- 



Relation to Deleterious Chemical Agents 453 

tack the grape-vine and there are at least half as many 
fungous diseases of the same plant. Numerous instances 
might be cited in which the number of fungous and insect 
pests of a particular crop is as great as those indicated. 1 
On the other hand, there are cultivated crops which re- 
quire very little consideration with respect to parasites of 
any kind. 

It is only in relatively recent times that spraying opera- 
tions have developed, especially spraying to prevent fun- 
gous diseases. This type of control has been in large part 
due to a careful investigation of the relations between 
plants and toxic solutions on the one hand, and to the de- 
velopment of effective spraying devices on the other. In 
nearly all cases protection against fungous diseases and 
insect pests is effected by covering the surfaces of fruit, 
leaves, and stems with a poisonous substance, which 
should, while relatively noninjurious to the host, prevent 
the effective germination and penetration of the spores, 
or kill the insects concerned. 

In controlling insects, it is to be remembered that there 
are two great classes with respect to the method of attack ; 
these are : — 

(1) Chewing insects which bite off and eat the vegeta- 
tive parts of the plant ; for example, cabbage worms, tent 
caterpillars, and potato beetles, which would be killed by 
poisons sprayed upon the plant. 

(2) Sucking insects, or those which get their food by in- 

1 Some estimates of the amount of damage annually sustained by the 
crops of the United States have been made, and taking as a basis the 
prices at which the crops actually sell, it seems to be demonstrated that 
the vast sum of one billion dollars may be aggregated. 



454 Plant Physiology 

serting a beak directly into the tissues, for example, plant 
lice and squash bugs. 1 

278. Destruction of weeds by poisons. — For years it 
has been more or less customary to employ salt for the 
destruction of weeds in the lawn or garden, or to suppress 
all plants growing in walks and playgrounds. It is only 
within recent years, however, that any special study has 
been bestowed upon the use of toxic solutions in the form 
of sprays as one of the recognized methods of weed control 
in lawns and cultivated fields. It is not a method which 
may be expected to replace the usual practices of clean 
cultivation, rotation, or pasturing, nor is it one which 
should lead the grower away from a close study of the root- 
ing and reproductive habits of weeds. 

1 The poisons or insecticides commonly employed for biting insects 
are such as Paris green, arsenate of lead, arsenite of soda, arsenite of lime, 
London purple, and hellebore. Of these, Paris green is by far the most im- 
portant. The use of this substance attracted general attention between 
1860 and 1870, when the Colorado potato beetle became an important 
factor in potato production. Shortly afterwards, the same mixture was 
employed throughout the South against the so-called army-worm of cot- 
ton, and it has since been used to give protection against an endless num- 
ber of biting insects. 

In employing the usual means of control against sucking insects, such 
substances as kerosene emulsion, miscible oils, whale oil soap, and lime- 
sulphur wash may be used, as well as methods of fumigation. The first 
three substances mentioned may be employed, with care, upon the foliage 
and growing parts. Miscible oils, carbolic acid, and relatively strong 
kerosene may be used only when the plant is in a dormant condition. 
Fumigation with tobacco smoke is common. The highly toxic vapor of 
hydrocyanic acid, prepared from potassium cyanide and sulfuric acid, 
has also been employed in the fumigation of trees under tents and with 
nursery stock in a dormant condition. It may also be used in the green- 
house with care, but special instructions are needed in any particular case. 

The effective use of chemical agents as protective measures against 
fungous diseases, dates from the discovery of Bordeaux mixture by Millar- 
det in France, 1883. Since that time, there has been organized through- 
out the United States and in foreign countries extensive methods of con- 
trolling these diseases. 



Relation to Deleterious Chemical Agents 455 

279. Deleterious substances employed. — Common salt 
is only slightly toxic, yet it has been much employed on 




Fig. 130. Greater ragweed in untreated field of wheat. [Photograph by 
H. L. Bolley.] 

account of its osmotic action. It may be used dry in 
roadways and other situations, and it has sometimes been 
effective in suppressing broad-leaved, delicate weeds in 



456 



Plant Physiology 



lawns, where it may be applied at the rate of from 3 to 6 
pounds per square rod. Crude carbolic acid possesses an 










Fig 131. Wheat in plat contiguous to that in Fig. 130, showing effect 
of iron sulfate spray on ragweed. [Photograph by H. L. Bolley.J 

objectionable odor, but it is very effective in killing 
vegetation in walks or courts. It may be sprayed upon 
the ground at a strength of 1 quart of the acid to 5 gallons 



Relation to Deleterious Chemical Agents 457 

of water. Waste formalin at the rate of 1 pound to 10 gal- 
lons of water may be used under similar circumstances. 

Copper sulfate and iron sulfate are, however, the two 
compounds which may be commercially employed with 
hand or power sprayers for the suppression of certain weeds 
in fields of grain or flax, and in large lawns. Copper 




Fig. 132. 



Wild mustard of size effectively reached and readily injured 
by the spray. 



sulfate is commonly used as a solution containing from 3 
to 5 per cent of the salt, — 12 to 20 pounds to 50 gallons of 
water. It is usually recommended to employ iron sulfate at 
a strength of 1-J to 2 pounds of the salt per gallon of water. 
280. Practicability of the chemical method. — The 
chemical method may be wisely employed under certain 
circumstances, as follows : — 



458 



Plant Physiology 



(1) Upon ground where no vegetation is desired— walks, 
playgrounds, courtyards, etc. 




Fig. 133. Natural growth of dandelions in an untreated lawn. [Photo- 
graph by H. L. Bolley.] 

(2) When particularly undesirable weeds are present in 
small spots, and the temporary suppression of all growth 
is not a serious objection. 



Relation to Deleterious Chemical Agents 459 

(3) When it is desired to suppress weeds or to prevent 
weed seeding during the maturity of a seed crop. 




Fig. 134. Lawn with dandelions, similar to that in Fig. 133, but treated 
with iron sulfate two weeks before blossoming. [Photograph by H. L. 
Bolley.] 

(4) When, during the growing season of the crop, a ma- 
jority of the undesirable weeds are more sensitive than the 
crop grown. 



460 Plant Physiology 

It is obvious that the use of chemical sprays for weed 
eradication in the field is dependent upon the resistance of 
the crop as compared with the weed and to the penetration 
of the chemicals employed. This method has been found 
especially applicable in growing cereal crops, grasses, flax, 
and peas. 1 

The plants which are killed are those whose surfaces are 
easily wet by the spray, but there are some plants, the 
common plantain (Plantago major), for example, which, 
although wet, is almost unaffected. Those which are not 
wet generally possess smooth glaucous leaves, or are pro- 
vided with a waxy bloom. In any plant the succulent 
or rapidly growing portions are more easily killed. Thus 
it follows that this means of eradication may be generally 
employed for plants with an indefinite habit of growth. 

LABORATORY WORK 

Toxic action. — ■ Determine the limiting eoncentrations of 
CuS0 4 and H 2 S0 4 for inhibition and growth of roots of corn 
and peas. Use the tumbler-culture methods employed in the 
study of mineral nutrients, or, if observations cover only a 
short interval of time, the germinating seeds may be pinned to 
the lower surfaces of corks covering the vessels employed, the 
roots projecting into the solutions. Make decinormal stock 
solutions of the toxic agents. With the CuS0 4 employ at least 

1 From a considerable number of experiments, it has been found that 
such plants as the following are more or less readily killed: bindweed, 
Canada thistle, dock, great ragweed, lamb's quarter?, mustard or char- 
lock, orange hawkweed, sow thistle, wild buckwheat, and wild radish. 
Weeds which have not been successfully combated without injury to the 
growing crop are such as bentgrass, bull-thistle, couch-grass, horse-tail, 
pigweed, and others. With iron sulfate Bolley has been able to hold 
the dandelion in check ; but on account of the perennial root, this plant 
is one of the most difficult to eradicate. 



Relation to Deleterious Chemical Agents 461 

N N 

four or five dilutions between and ; while with 

10000 300000 

N N 

H0SO4 prepare dilutions ranging from to • 

800 8000 

Effect of insoluble particles. — After following carefully the 
discussion in the text, determine, through cultures in tumblers, 
the concentration of CuS0 4 and of H 2 S0 2 which will inhibit and 
permit growth in (1) granulated quartz or infertile sand, and in 
(2) a rich garden loam. In these experiments use, in each case, 
sufficient of the solution to moisten the substratum approxi- 
mately, and in comparative experiments the same amount of 
solution should be used. Permit the experiments to run only 
one week, and watering will not be required. 

Toxic agents and foliage. — With a hand spray or atomizer 
treat the foliage of convenient plants in the greenhouse or field 
with 3 per cent copper sulfate, and 5 per cent iron sulfate. 
Study the comparative effects. Cereals, grasses, carnations, 
and onions may be taken as types of foliage not easily wetted, 
while dandelions, mustard, beans, and peaches will furnish suit- 
able contrasts. 



References 

Bolley, H. L. Weed Control by Means of Chemical Sprays. 

N. Dak. Agl. Exp. Sta. Bui. 80 : 541-574, pi. 11-29, 

1908. 
Brown, A. J. The Selective Permeability of the Covering of the 

Seeds of Hordeum vulgare. Proc. Roy. Soc. 81 B : 82-93, 

1909. 
Clark, J. F. On the Toxic Effect of Deleterious Agents on the 

Germination and Development of Certain Filamentous 

Fungi. Bot. Gaz. 28 : 289-327, 378-404, 1899. 
Crocker, W., and Knight, L. I. Effects of Illuminating Gas and 

Ethylene upon Flowering Carnations. Bot. Gaz. 46 : 259- 

276, 1908. [Compare also, Knight, Rose, and Crocker, 

Science, N. S. 31 : 635-636.] 



462 Plant Physiology 

Davenport, C. B. Experimental Morphology. 1 : 1-52. 
Duggar, B. M. Fungous Diseases of Plants. Pp. 85-92, 1909. 
Heald, F. D. On the Toxic Effect of Dilute Solutions of Acids 

and Salts on Plants. Bot. Gaz. 22 : 125-153, 1896. 
Jensen, C. H. Toxic Limits and Stimulation Effects of some 

Salts and Poisons on Wheat. Bot. Gaz. 43 : 11^4, 34 figs., 

1907. 
Jones, L. R. Chemical Weed Killers or Herbicides. Bailey's 

Encyclopedia of American Agriculture. 2:115-118, 1908. 

[Also Vt. Agl. Exp. Sta. Report. 12 : 182-188, 1899.] 
Kahlenberg, L., and True, R. H. On the Toxic Action of Dis- 
solved Salts and their Electrolytic Dissociation. Bot. Gaz. 

22 : 81-124. 
Lodeman, E. G. The Spraying of Plants. 399 pp., 92 figs., 1896. 
Loew, O. Ein natiirliches System der Gift-wirkungen. 1893. 
On the treatment of Crops by Stimulating Compounds. 

Bui. Coll. of Agl. Imp. Univ. Tokyo. 6:161-175, 1904. 
Richards, H. M. Die Beeinflussung des Wachsthums einige 

Pilze durch chemische Reize. Jahrb. f. wiss. Bot. 30 : 

665-688, 1897. 
Schreiner, O., and Reed, H. S. Certain Organic Constituents 

of Soils in Relation to Soil Fertility. Bur. of Soils, U. S. 

Dept. Agl. Bui. 47 : 52 pp., 5 pis., 1907. 
Stone, J. L. Spraying for Wild Mustard. Cornell Agl. Exp. 

Sta. Bui. 216: 107-110. 
True, R. H., and Oglevee, C. S. The Effects of the Presence 

of Insoluble Substances on the Toxic Action of Poisons. 

Bot. Gaz. 39 : 1-21, 1905. 



CHAPTER XIX 
VARIATION AND HEREDITY 

In organic variation, our interest centers on the 
mechanism and forces concerned in the adjustment of 
an organism to its environment. Variation signifies 
change, and may be evidence of past and present influences ; 
heredity gives a record of the past and certain promises 
for the future. Both of these are important aspects of 
evolution. 

Many theories have been advanced in explanation of 
the facts of variation and heredity. No single theory re- 
ceives at present universal sanction. Every reasonable 
hypothesis merits careful consideration, and the present 
widespread interest in experimental evolution makes it 
particularly desirable to view new facts in an unpreju- 
diced light. The limited scope of this book makes it 
possible to include only a brief presentation of some of the 
important facts and views, as an introduction to the sub- 
ject; the fuller theoretical treatment and application 
must be sought in the literature. 

VARIATION 

The capacity for variation is a fundamental possession. 
It is universal with living organisms. Though heredity 
463 



464 Plant Physiology 

characteristics of parents are transmitted to the offspring, 
yet not all individual characteristics of all ancestors are 
transmitted. The offspring ma} r exhibit modification. 
This modification may be evident under constant condi- 
tions, or it may occur in response to environmental changes. 
It may be an acquirement manifest merely during the 
life of the organism, or it may be innate and trans- 
missible. Every organism possesses an individuality. 

281. Individuals and species. — Individuals which re- 
semble one another closely and which have a common 
origin may collectively constitute what may be called a 
race, variety, or species. Our ideas of such groups are 
based upon a study of individuals (few or many, small or 
large populations) and naturally center about average 
examples. We recognize, however, certain extremes ; in 
fact, there are multitudinous variations, for there may be 
as many extremes as characters, or character combinations. 
These extremes, perhaps, have in many cases so insensibly 
entered into other recognized varieties that opinions would 
differ in determining to which variety a particular individual 
should be attached. Some varieties, on the other hand, 
may stand apart with sharply differentiated characters; 
within these, individuals may also differ perceptibly 
among themselves. In any case, a group of individuals, 
such as a race, variety, or species, is in a measure a theo- 
retical average with respect to characters, and is made 
up of a series of individuals showing in the different 
characters considerable fluctuation. 1 

1 All organisms resembling one another closely must look alike to 
the inexperienced eye, — to the eye unfamiliar with the group. Upon 
close inspection and measurement, however, relatively wide differences 



Variation and Heredity 465 




Fig. 135. Variation in the leaves from a single bud of sassafras. 
2h 



466 Plant Physiology 

282. Fluctuating variation. — The minor differences 
which all individuals of any population exhibit are com- 
monly fluctuating, or continuous, variations. The ideal, 
or type, which these individuals approach is an average 
individual, with reference to a number of characters. If 
many individuals be examined with respect to any one 
character, the result may be given in the form of a curve of 
variation. An examination, for instance, of a population 
of the common field daisy would disclose a variation in 
the number of ray flowers ; thus there might be from 10 to 
20. This variation in number represents the range, and 
the numbers 10, 11, 12, 13, etc., constitute the variates or 
classes. Perhaps the majority of the population would 
have the same number of ray flowers, say 15, which class 
would then represent the mode, or class of greatest fre- 
quency. In a normal curve there would be a diminishing 
frequency towards both higher and lower classes. Quete- 
let has shown that this curve, in the main, corresponds 
with the law of probabilities, or curve of frequency of 
error. 

invariably appear. The tomato is an excellent example of variability. 
Little more than a century ago it was introduced as a vegetable. To- 
day its varieties are numbered by the hundred, and there are a great 
many well-defined types and forms of fruit, characters of leaf, size. etc. 
We are thus sure that by one means or another variation has been effected, 
and in a marked degree. Once several strains or varieties are developed, 
hybridization is the greatest possible source of variation, or multiplica- 
tion of forms. 

The use of score cards in judging corn, apples, and other farm and 
horticultural crops draws special attention in a practical way to standards 
among economic plants, and at the same time to departure from the 
standards, — to variation. The producer is likely to have in mind as 
an ideal the best or most highly developed type of any variety or strain, 
and thus in the work of selection he may constantly depart from the 
old ideal in the direction of a new and improved strain. 



Variation and Heredity 467 

The symmetrical curve shows the highest frequency, or 
mode, in the center, but not infrequently the mode is 
considerably shifted from one side to the other, giving 
skew curves. Again, multimodal curves occur, and many 
other subsidiary forms have been found to prevail in cer- 
tain species or races. 

283. Darwin's theory of natural selection. — Through 
his wide experience with living things Darwin was thor- 
oughly conversant with the existence of fluctuations in 
nature. It was apparent that growers select, isolate, and 
breed desirable forms or individuals, excluding or destroy- 
ing those undesirable. Such a process appears to have led 
to the origination of new breeds and races. Darwin saw 
in nature similar forces yielding similar results ; viewing 
the problem, therefore, in the uncontrolled or natural 
environment, he formulated the following ideas : (1) any 
individual variation, slight or considerable, which enables 
the organism possessing it to succeed or maintain itself 
better than its neighbor will have a strong chance of be- 
coming perpetuated ; (2) more seeds are produced than 
can grow again unto seedage, more organisms enter upon 
life than can be reared ; (3) those less well equipped for 
life's struggle succumb, and there is manifest a powerful 
process of Natural Selection. 

He would seem to have maintained that the main line of 
evolutionary progress and change lies in the natural or 
artificial selection of relatively minute variations ; that is, 
natural selection lays hold upon fluctuating variations. 
There is constant variation, hence there is constant change, 
or evolution. Wide variations arising suddenly, termed 
sports, or discontinuous variations, were apparently 



468 Plant Physiology 

regarded by Darwin as of less importance in evolutionary 
progress (see section 287). 

In most of the present-day discussions respecting evo- 
lution, natural selection is recognized as a potent force ; 
but great diversity of opinion prevails with regard to the 
magnitude of the variations by means of which progress in 
selection is maintained. 

284.' Rate of increase. — A study of the theoretical 
rate of increase of many organisms involves numbers which 
are not easily grasped. A single tobacco plant may 
produce from 500,000 to 1,000,000 seed. At the minimum 
production mentioned the second year there would be 
250 billion seed, and the product the third year would 
be expressed by eighteen figures. At the rate of one seed 
per square foot, this number would plant the surface of 
the earth several hundred times over. A vigorous 
specimen of the common dandelion under observation 
produced in a season about thirty flower-heads, each 
averaging about 300 seed, or 9000 seed for the season. 
In this case, there would result at the end of the fourth 
season 6,561,000,000,000 seed. 1 

Assuming the capacity of an organism to vary, the power 
of the environment to suppress and exterminate unfitness 
makes it a very strong factor in determining the nature. of 

1 One instance from the animal side may be cited (adapted from 
Jordan and Kellogg, "Evolution and Animal Life," p. 59), that of the 
quinnat salmon of the Columbia River. This is a prolific fish whose 
eggs and young are poorly protected and consequently devoured by 
numerous greedy enemies. A female will ascend the river when four 
years old and deposit about 4000 eggs, subsequently dying. Allowing 
for 50 per cent of males and the normal period of maturity of females, 
only five generations would be required, should all individuals survive, 
for this fish to occupy far more than the volume of the sea. 



Variation and Heredity 469 

those creatures which survive. Since, moreover, the or- 
ganisms which survive respond to the influence of the 
environment, be it much or little, the stamp of environ- 
ment is ultimately borne by every living thing. This 
does not imply, however, that the environment stimulates 
change in the direction of fitness for the particular en- 
vironment, yet a strictly physico-chemical explanation 
would perhaps demand this. 

285. Fluctuating variation and the origin of varieties. — 
The artificial selection of fluctuating variations has been 
the basis of great improvement, or of the maintenance of 
standards, in many cultivated crops. The sugar content 
of good varieties of the beet has been increased from be- 
tween 8 and 10 per cent to from 14 to 18 per cent. Many 
deny permanence to this type of selection, and much experi- 
mental work appears to be in progress, designed to throw 
light upon the question. 

Punnett says: " The small fluctuating variations are not 
the materials on which selection works. Such fluctuations 
are often due to conditions of the environment, to nutri- 
tion, correlation of organs, and the like. There is no in- 
disputable evidence that they can be worked up and fixed 
as a specific character.'' Castle, speaking of the heredity 
of fluctuations, says, "It is an exceedingly difficult and 
slow process, and its results of questionable permanency." 

Physiological modifications in corn. — Unusually inter- 
esting experiments in corn variation and breeding have 
been conducted at the Illinois Experiment Station. In this 
case variation in chemical content with respect to high 
protein and low protein, also high oil and low oil, has been 
made the subject of study, and the results for a ten-year 



470 



Plant Physiology 



period have been reported. A white dent corn was used, 
and to this the name Illinois has been given. In protein 
content the 100 ears of seed corn used the first year varied 
from 13.87 to 8.25 per cent. From the table in the note 
below l it will be seen that the ten years of selection sufficed 
to bring the average of the plat (1906) in high protein 
(14.26 % ) above the best ear in the first crop (13.87 % ), and 
the average in the low-protein plat (8.64 % ) was practi- 
cally as low as the lowest ear (8.25 % ). The yearly 
averages in the high-oil and low-oil breeding show a varia- 
tion often more striking than in the case just cited, as 
shown in the footnote. 

1 Ten Generations of Breeding Corn for Increase and Decrease 
of Protein 



Year 




High-protein Plot, 

Average Per Cent 

Protein 


Low-protein Plot, 

Average Per Cent 

Protein 


Difference 

between 

Crops, 

Per Cent 




In Seed 
planted 


In Crop 
harvested 


In Seed 
planted 


In Crop 
harvested 


1896 
1897 
1898 
1899 
1900 
1901 
1902 
1903 
1904 
1905 
1906 








12.54 
12.49 
13.06 
13.74 
14.78 
15.39 
14.30 
15.39 
16.77 
16.30 


10.92 
11.10 
11.05 
11.46 
12.32 
14.12 
12.34 
13.04 
15.03 
14.72 
14.26 


8.96 
9.06 
8.45 
8.08 
7.58 
8.15 
6.93 
7.00 
7.09 
7.21 


10.92 
10.55 
10.55 
9.86 
9.34 
10.04 
8.22 
8.62 
9.27 
8.57 
8.64 


.00 
.55 
.50 
1.60 
2.98 
4.08 
4.12 
4.42 
5.76 
6.15 
8.62 



The relatively high protein content in the product harvested in 1901 is 
evidently the result of early maturity of the seed during that remarkably 
dry season. 



Variation and Heredity 



471 



Summarizing the extreme variations in these qualities at 
the beginning and close of the periods, the table on the 
next page is suggestive. 

286. Pure lines. — Johannsen has employed the term 
" pure line " to denote the offspring of a single individual 
produced by self-fertilization (thus isolating a type, 
genotype). With pure lines he has conducted extensive 
selection experiments, and the results for quantitative 
characters stand in contrast to those obtained by selec- 
tion in an ordinary population. Selection within a pure 
line, from modal individuals and from those showing the 
greatest deviation, yield offspring the averages of which 
are the same. According to this, selection within the 



Ten Generations of Breeding Corn for Increase and Decrease 
of Oil 





High-oil Plot, 


Low-oil Plot, 






Average Per Cent Oil 


Average Per Cent Oil 


ENCE BE- 


Year 














In Seed 


In Crop 


In Seed 


In Crop 


Crops, Per 




planted 


harvested 


planted 


harvested 


Cent 


1896 . . . . 





4.70 





4.70 


.00 


1897 








5.39 


4.73 


4.03 


4.06 


.67 


1898 








5.20 


5.15 


3.65 


3.99 


1.16 


1899 








6.15 


5.64 


3.47 


3.82 


1.82 


1900 








6.30 


6.12 


3.33 


3.57 


2.55 


1901 








6.77 


6.09 


2.93 


3.43 


2.66 


1902 








6.95 


6.41 


3.00 


3.02 


3.39 


1903 








6.73 


6.50 


2.62 


2.97 


3.53 


1904 








7.16 


6.97 


2.80 


2.89 


4.08 


1905 








7.88 


7.29 


2.67 


2.58 


4.71 


1906 








7.86 


7.37 


2.20 


2.66 


4.71 



472 



Plant Physiology 



pure line cannot change the averages, or shift the mode. 
These results are most suggestive and important, and 
the principle has been confirmed by Jennings and others ; 
but many additional data will be required before this 
type of behavior is recognized as of general significance. 

Table showing Extent of Variation 



Type of Breeding 


Best Ear 


Extreme Variation 


High protein 


per cent 


per cent 


1896 


13.87 


5.62 original seed 


1906 


17.67 


10.94 tenth year selection 


Low protein 






1896 


8.25 




1906 


6.73 




High oil 






1896 


6.02 


2.18 original seed 


1906 


8.51 


6.91 tenth year selection 


Low oil 






1896 


3.84 




1906 


1.60 





287. Mutations. — A sudden variation which is fully 
transmissible has been called by Dc Vries a mutation. It 
is a discontinuous variation or hereditary saltation. 
Mutation phenomena have become prominent in variation 
studies since the publication by De Vries of his Mutation 
Theory. Already the term has been loosely employed, but 
in general it has the special significance above indicated. 
De Vries advanced the view that all evolutionary progress 
is based upon the occurrence of mutations. His conclu- 



Variation and Heredity 



473 




474 Plant Physiology 

sions were primarily the result of extensive studies upon 
the evening primrose ((Enothera Lamar ckiana) , and upon 
a general review of available information respecting both 
the origin of domesticated varieties, and the behavior of 
organisms in nature. Even though some hesitate to 
accept all the conclusions arrived at for the (Enothera 
mutants, the principle of mutation has been accepted by 
many students of evolution as a working hypothesis; 
and many are now endeavoring to determine the extent, 
frequency, and behavior of such mutants. 

According to the current view a mutation may be a 
variation relatively great or small, involving a single unit 
character or a group of such characters. The mutation 
is often of greater, but may be of lesser, extent than the 
fluctuation, and the existence of the two types together 
may lead to much confusion. Far more careful analytical 
work will be required before it may be possible fairly to 
estimate the respective value in evolution of mutation and 
fluctuation, or indeed properly to distinguish types of 
variation. There can be no doubt that . striking cases 
are on record of the occurrence of saltation ; but it is 
obvious that the extreme supporters of the mutation 
principle, by the definition and explanation of the term, 
actually exclude the possibility of any such phenomenon 
as transmissible fluctuation. 

Tower and Blaringhem working with beetles and with 
corn respectively have reported some results particularly 
interesting in this connection. After demonstrating the 
effect of environment in producing continuous variation 
in Chrysomelid beetles within the range of the species, 
Tower reports a striking case of difference in behavior. 



Variation and Heredity 475 

The offspring of certain pairs of beetles showing precisely 
the same variation in spot characters were compared. 
The offspring of one pair transmitted the variation, while 
those of other pairs were unable to do so, varying 
toward the mean of the species. Blaringhem was able 
through a variety of injuries to produce certain abnor- 
malities of corn flowers, especially the production of 
grains in the staminate inflorescence. This abnormality 
was not generally transmissible, yet it was transmitted 
in a few cases, and even the degree of transmission was 
found to be variable. 

288. Mutation and crop improvement. — The principle 
of mutation appears to be particularly important in crop 
improvement. Taken in conjunction with the facts of 
alternative inheritance, subsequently discussed, it directs 
attention to uncommon individuals and types, and to the 
greater probability of securing permanent and immediate 
improvement by the isolation and breeding of such forms. 

No one has contributed more to the method and results 
of selection work than Nilsson, the Swedish investigator, 
whose work has been made a special study by De Vries. 
Nilsson devoted particular attention to the cereals, and 
his method of selection was founded upon the discovery 
that " a protean group of types was found to constitute 
each so-called variety. These types were seen to be differ- 
ent from one another in a previously unsuspected degree, 
covering a range of variability adequate to comply with 
almost all the needs of practice." 1 

The practical success of the work of Burbank and others 
seems to rest upon a careful search for variable forms; 

1 De Vries, "Plant Breeding," p. 68. 



476 Plant Physiology 

the utilization of large numbers, in order that there may be 
more chance for variation ; and in the detection and iso- 
lation of the unusual individual or type. 

HEREDITY 

In the production of plants there are two primary re- 
quirements, — there must be (1) the seed or propagative 
parts, and (2) certain favorable conditions for growth 
and reproduction. The one is a biological mechanism 
which has behind it ages of ancestors determining specifi- 
cally or racially what type of plant there shall be; the other 
is a complex of physical and chemical factors conditioning 
what kind of individual there shall be. The embryo 
plant possesses its particular hereditary possibilities, and 
it is encompassed by an environment which sustains it or 
subjects it. Heredity and environment are therefore 
forces closely linked together in biological investigation. 
Environment is important in molding heredity, and 
heredity constantly affects the method of response to 
environment. All biologists agree that either structural 
or functional adjustments to environment may ultimately 
become hereditary; but a chief tenet of Weismannism is 
that no change is hereditary which does not affect the 
germ cells. 

There is at present great activity in the study of hered- 
ity, a manner of cell behavior which we may now tenta- 
tively define as being concerned with the transmission 
through successive generations of racial and individual 
characters. A fundamental study of transmission is 
properly termed Genetic Physiology, or simply Genetics. 



Variation and Heredity 477 

It finds direct practical application in all the practices 
of plant and animal breeding. 

289. Nonsexual reproduction and heredity. — In non- 
sexual reproduction a part of an individual reproduces a 
new individual, <and the latter commonly resembles the 
former as closely as environment or the conditions of its 
growth will permit. In this case we may scarcety speak 
of heredity or transmission in the usual sense ; yet it is 
important to recall that whatever the nature of the part 
used for such vegetative multiplication, it retains the racial 
or specific characteristics of the plant from w T hich derived. 
The propagative part or scion usually includes one or 
more buds. Reduced to the lowest terms conceivable, it 
might be a single vegetative cell. In at least two cases 
among flowering plants an epidermal cell may develop a 
bud, and this bud reproduces the plant. In any case it is 
remarkable that a single cell, or even a group of meriste- 
matic cells, should be able to reproduce so completely 
all the qualities of a complex adult. Bud variations may 
occur in plants propagated by nonsexual means, yet most 
clonal varieties are said to be fairly constant. 

290. Sexual reproduction and heredity. — In sexual 
reproduction, individuals contribute characteristics through 
the two gametes or uniting cells, and through this union 
two lines of ancestry are united in one organism. As 
already noted, in the angiosperms these gametes are a 
nucleus, with little or no accompanying cytoplasm, from 
the pollen tube and an egg cell in the ovule ; these micro- 
scopic, protoplasmic units must carry all the characteris- 
tics entering into the organism. This organism will not 
commonly resemble in absolute detail either parent, nor 



478 Plant Physiology 

will it be an exact mean between the two, as a rule, espe- 
cially where the parents show contrasting characters. It 
may show distinctive characteristics of both, perhaps some 
characteristics evident only in more distant ancestors, 
and others which may seem to have been modified, or 
which may appear to be entirely new. 

The problems respecting the method of transmission 
are important, and the theories offered in explanation are 
both interesting and valuable ; but the physiological 
picture is as yet more or less indefinite. The evidence 
derived from a minute study of the cell, or C3 r tology, may 
be of assistance, but it is not to be expected, in general, 
that any single characteristic of the organism will leave a 
special morphological imprint upon the nuclear structure. 

291. The early studies. — The early studies upon 
heredity yielded many valuable observations, yet they 
were disappointing with regard to definite results and to 
the development of special methods for attacking the 
general problem. Kolreuter, Knight, Gartner, and others 
accumulated interesting data. Galton employed statis- 
tical methods, and his studies of pedigree records led him 
in 1897 to announce his famous " law of ancestral heredity." 
By this hypothesis, assuming unity as the total hereditary 
possession of any organism, he assigned diminishing values 
in a geometrical series (the total approaching unity) to 
the ancestors in preceding generations, from parents to 
those more remote, averaging as follows : parents one half, 
grand-parents one fourth, great-grand-parents one eighth, 
etc. This conception may be regarded, perhaps, as an 
expression of the practical results of complex hereditary 
influences, through many generations ; but from it we seem 



Variation and Heredity 479 

to get no indication of a particular method in heredity, 
and no possible analysis of the independent characters 
concerned. As a matter of fact, in Mendelian inherit- 
ance, subsequently discussed, it will be apparent that 
certain ancestors may contribute nothing, or some few 
characters only. 

292. Types of inheritance. — There are apparently 
several distinct types of hybrid inheritance, such as 
blended, intensified, mosaic, heterogenous, and alternative. 
Three of these may be briefly characterized as follows : — 

1. Blended inheritance. — The crossing of forms dis- 
tinct with respect to any character yields offspring pos- 
sessing this character to a degree intermediate between 
the parents. Some cases of apparent blends are ques- 
tioned, and much more study of this type is required. 

2. Intensified inheritance. — The crossing of forms dis- 
tinct with respect to any character (e.g. size) yields off- 
spring possessing that character more highly developed 
than either parent. Certain plums produced by Burbank 
are apparently of this type. 

3. Alternative inheritance. — The crossing of forms 
distinct with respect to any character yields offspring 
which resemble one parent only ; but the hybrid nature of 
this first generation is shown by its offspring. In the latter 
there is segregation, as explained later, in such manner 
that each character appears practically unchanged in a 
part of the offspring. 

293. Recent studies. — The newer studies upon heredity 
practically began with the new century, and with the 
rediscovery of work done nearly half a century earlier. 
These investigations are in a large measure concerned 



480 Plant Physiology 

with alternative inheritance in hybrids, that is, with 
offspring from parentage showing contrasting characters. 
The work has resulted from a clear appreciation of certain 
fundamental observations. A single case may serve to 
typify the simplest form of the problem : Bearded wheat 
is crossed with beardless. Will the progeny be bearded, 
beardless, or intermediate? What will happen in succeed- 
ing generations? For the development of this line of 
inquiry, we are indebted first of all to Gregor Johann 
Mendel, — Priest and later Pralat of the Konigskloster 
of Briinn — and to De Vries, Bateson, Castle, Correns, 
Tschermak, and many others. Mendel's important con- 
tribution was published in I860, but it attracted no atten- 
tion and was practically lost to the scientific world until 
rediscovered in 1900. 1 

294. Mendel's experiments. — Mendel had followed 
carefully the work of such predecessors in this line of in- 
vestigation as Kolreuter, Knight. Gartner, and others. 
He was particularly interested in what has been charac- 
terized as the constant appearance of the same hybrid forms 
when any two species are crossed. He sought to deter- 
mine the number of such forms which may arise, the con- 
duct of these in the succeeding generations, and the rela- 
tions of the forms one to another from a numerical or 
statistical point of view. He had an unusually clear idea 
of the indications which should be possessed by the species 
employed in crossing in order to demonstrate the points 
in a definite manner. He declared that species to be 

1 For a comprehensive, bibliographical sketch of Mendel, the student 
should read the notice of him in Bateson's "Mendel's Principles of Hered- 
ity," pp. 304-316. 



Variation and Heredity 481 

crossed should possess differentiating or contrasting 
characters ; that the hybrid offspring should offer the 
possibility of being readily protected from foreign pollen ; 
and that the offspring should be, with respect to fertility, 
unaffected by the inbreeding process necessarily pursued. 

He found in the common garden pea (Pisum sativum), 
and other related forms, promising material for his work. 
After testing for two years the constancy of thirty-four 
varieties, proceeding with great care, he selected those 
which showed well-defined contrasting characters, — in all, 
seven combinations. We may consider three of the typical 
cases with a single differential character-pair (later termed 
simple allelomorph, or allelomorphic pair) as follows : — 

(1) Difference in color of cotyledons; yellow vs. green. 

(2) Difference in the color of the seed coats ; white vs. 
colored. 

(3) Difference in size ; tall vs. dwarf. 

Like Darwin and others who were interested in a more or 
less similar line at the same time, he recognized the neces- 
sity of dealing with large numbers in order that individual 
errors might be avoided as far as possible. From the 
crosses obtained with the strains showing the contrasting 
characters above mentioned, he found that with respect to 
these characters all of the hybrids resembled one of the 
parents; that is, there was no blending of these qualities. 
The character which appeared in the hybrid of the first 
generation, known as the F x generation, was termed the 
dominant of the pair, and that character which was veiled 
or latent in the F x was termed the recessive. In the three 
cases above the dominant characters were yellow cotyle- 
dons, colored seed coat, and tall habit. No transitional 
2i 



482 



Plant Physiology 




Variation and Heredity 483 

forms were found in the hybrid generation, and in these 
cases reciprocal crosses gave in the F x generation plants 
entirely alike. 

A summary of the results of the F 2 generation is of 
special interest. Upon planting the seed of the F x genera- 
tion there resulted, in the first case, 258 individuals; 
and these yielded in the F 2 generation 8023 seeds, of which 
6022 were yellow and 2001 green. In other words, the 
relation of yellow to green was 3.01 : 1. Where color in 
the seed coats was a contrasting character there were 929 
plants in the F 2 generation, of which 705 produced colored 
seed coats, correlated also with color of blossoms ; and 
224 produced white seed coats, correlated with white 
flowers. In this instance the proportion was 3.15 : 1. In 
the third case, there were in the F 2 generation 1064 plants, 
of which 787 were tall and 277 dwarf, or a ratio of 2.8 : 1. 

In each case one fourth of the individuals, showing the 
recessive character, breed true in all subsequent generations, 
that is, in F 3 , F 4 , etc. In analogous manner one fourth of 
the whole number of F 2 individuals (one third of the ap- 
parent dominants) breed true as dominants. The re- 
mainder, one half, are hybrid dominants and break up in 
the F 3 generation exactly as did the whole number in the 
F 2 generation. This method pertains through successive 
generations. It is therefore apparent that the F 2 
generation may be represented thus : D + 2 D(R) + R, 
in which D represents dominants, R recessives, and D(R) 
hybrid or impure dominants, in which only the dominant 
character is evident. The latter are here indistinguishable 
from dominants, except as they show segregation in the 
next generation. 



484 Plant Physiology 

295. Purity of the gametes. — As a result of these 
hybridization studies, Mendel naturally developed his 
theory of the purity of the gametes, founded substantially 
in this way: In the case of the hybrid between a tall (T) 
and a dwarf (S) pea, for example, the male gametes in 
equal number will carry the character tallness or dwarf- 
ness, never both ; and so also with the egg cells. Assum- 
ing large numbers, these gametes, uniting by the law of 
chance (without selective fertilization), would yield, tall- 
ness being dominant, — 

Tall with tall or TT (homozygote) 

Tall with dwarf or T(S) (heterozygote) 

Dwarf with tall or (S)T=T(S) (heterozygote) 

Dwarf with dwarf or SS (homozygote) 
from which we get T+2 T(S)-f-S. The i ssential features 
of Mendelism are dominance and segregation, and these 
phenomena are sufficiently important to-day to receive 
unusual attention. It lias been well shown that many 
Mendelian character pairs may be expressed conveniently 
in terms of presence and absence of a single character. 
Either presence or absence may be dominant. The 
idea of the 1 purity of the gametes requires modification, 
at least with respect to certain characters. 

296. Results of segregation. — In corn yellow kernels 
are dominant over white. Indicating the yellow by Y, 
the white by W, and the white in the hybrid, where it is 
inevident, by (W), the following diagram indicates the 
method of segregation in five generations, and the relative 
number of pure dominants, recessives, and hybrid domi- 
nants in the hypothetical case where the rate of increase 
is fourfold : — 



Variation and Heredity 



485 



Genera- Dominants Hybrid 
tion ffl x Dominants 




Recessives 


Parent (dominant yellow) Y X 

t 


W (recessive white) 


1 








Fj Y (W) 
t 




1 


Y (W) + (W) Y 






(W)Y 


or 
F 2 Y 2Y(W) 

t 






W 


i 


2 Y (W) + 2 (W) Y 


or 
F„ 4 Y 2 V 4 V CW) 






2 W 4 W 


f 




1 








4Y (W)+4 (W) Y 




4 W 




or 
Fa 16 Y 8 Y 4 Y 8 Y CW) 




8 W 16 W 


t 




1 


W 


16 W 




| 8 Y (W) + 8 (W) Y 

or 
F 5 64 Y 32 Y 16 Y 8Y 16 Y (W) 8 


32 W 64 AV 


120 Y 16 Y (W) 




120 W 



Upon the principle of the purity of the gametes and the 
combination of these according to the law of chance, it 
becomes a simple mathematical problem to determine the 
number of combinations resulting from a cross in which 
two or more character pairs are considered. 



486 



Plant Physiology 



297. Tomato characters. — To accord with Mendelian 
results every plant may be regarded as made up of a cer- 




Fig. 138. Hybridization of tomatoes: parental types (top row), Honor 
Bright and Yellow Pear ; F, generation (2d row) quite uniform : F 2 
generation (3d and 4th rows) showing segregation. [Photograph by 
H. L. Price.] 

tain number of unit characters, which may be contrasted 
in different forms or varieties. Price and Drinkard have 
determined thirteen such alternative character pairs 



Variation and Heredity 



487 



for the tomato. These may serve as a further example 
of unit characters, and they are listed below, the dominant 
unit of each pair being placed in the middle column : — 

Fruit Shape : Spherical or Round . Pyriform 

Two-celled .... Many-celled 
Roundish-conic . . Roundish-compressed 

Fruit Color : Red Fruit Pink Fruit 

Red Fruit Yellow Fruit 

Pink Fruit .... Yellow Fruit 

Yellow Fruit Skin . . Transparent Fruit Skin 

Fruit Surface : Smooth Pubescent 



Foliage : Normal or Cut Leaf . 

Pimpinellifolium Leaf 

Green Leaf .... 

Normal or Smooth 

Leaf Surface . . . 

Stature : Standard Stature . . 



Potato Leaf 
Normal Leaf 
Yellow Leaf 

Rugous Leaf 

Dwarf Stature 



298. Chromosome relations. — A material basis for 
certain types of inheritance, especially Mendelian phe- 
nomena, seems to be found in the behavior of the chromo- 
somes. The reduction division, occurring in seed plants 
in the formation of the microspores (ultimately pollen) 
and megaspores (ultimately embryo-sac) is the important 
stage.. In this the 2x (somatic or diploid number of 
chromosomes is reduced to the x (gametic or haploid) 
number. It is generally held that the essential features 
of this division are two : (1) close association in pairs 
(bivalent chromosomes) of paternal and maternal chromo- 



488 



Plant Physiology 



somes, in which association, particularly, mutual character 
influences may find explanation; and (2) separation of 
the previously associated chromosomes (members of a 
pair), one to each of two daughter nuclei, thus segregating 




Fig. 139. Hybridization of tomatoes : same fruits as shown in preced- 
ing figure, in section. [Photograph by H. L. Price.] 

maternal and paternal characters. A comprehensive 
discussion of cytological relations is entirely beyond the 
scope of this work. 

299. Selection. — Selection of forms for hybridization 



Variation and Heredity 489 

requires intelligent care. Hybridization is primarily 
useful in order to effect the combination of desirable 
characters and the elimination of undesirable ones. Never- 
theless, it is generally agreed that in some cases of alterna- 
tive inheritance there may be left an influence of the cross, 
so that the characters may not reappear in their original 
purity. Moreover, as a result of hybridization latent or 
reversionary characters may appear, and crossing has a 
tendency to intensify variability. 

A knowledge of Mendelian behavior has necessitated a 
change in the methods of selecting hybrid offspring. In 
alternative inheritance there can be no selection in the F x 
generation. In the F2 generation selection for the pure 
recessives may be made. However, since homozygous 
dominants usually distinguish themselves from hetero- 
zygous dominants only in the absence of segregation in 
subsequent generations, it is necessary to isolate individ- 
uals and to test these in breeding plots. When several 
character pairs are involved, all of which require considera- 
tion, selection with respect to dominant characters may 
become very complex and tedious. 

LABORATORY WORK 

Variation. — Utilizing convenient plants in the greenhouse or 
in the field make a careful study of variation with respect to 
single characters readily enumerated, or measured. The number 
of ray flowers, or of bracts in certain composites, the number of 
flowers in a cluster, the number of eyes upon potatoes, the num- 
ber of leaflets in compound leaves, the difference in weight of 
grains or corn, and many other similar characters may be em- 
ployed. In such cases the counts or measurements should be 
made within the single variety. Determine the classes of 



490 



Plant Physiology 



variation, construct curves and prepare a report which shall 
include the results of the study made, together with an abstract 
of one or more of such papers as the following : — 

Davenport, C. B. Statistical Methods. Pp. 19-41. 
Davenport, E. Principles of Breeding. Pp. 681-703. 
Harris, J. A. Amer. Nat, 43 : pp. 350-355 ; ibid., 44 : pp. 19-30. 
Ludwig, F. Biometrika. 1: pp. 11-29. 
Pearson, K. Grammar of Science. Pp. 381-402. 
Shull, G. H. Amer. Nat. 36 : pp. 111-152. 

Compare, if possible, the curves of variation with respect to any 
character in two populations grown under dissimilar conditions. 




Fig. 140. Dahlia flowers with bursting anthers. [Photograph from 
Bureau Plant Industry.] 

Heredity. — Laboratory work upon heredity covering only 
one or two periods may be only suggestive, or may give opportu- 
nity for the presentation of materials which may be subsequently 
worked up as a report. Practical studies are preferably confined 
to the following : — 



Variation and Heredity 



491 



1. A study of the methods and manipulation of crossing, con- 
sulting such references as — 

Bailey, L. H. Plant Breeding. Pp. 344-358. 
Oliver. Bureau of Plant Industry. Bui. 167 ; 39 pp. 




Fig. 141. Dahlia flowers after depollination by means of a stream of 
water. [Photograph from Bureau Plant Industry.] 

2. The results of crossing. In the latter case parents, to- 
gether with first and second generation crosses, should be com- 
pared with respect to characters, following the suggestions in 
sections 294-297. Fresh material is most desirable, but if it 
may not be had at the time desired, alcoholic or dried material 
will serve adequately for the study of many characters. 



References 

Bailey, L. H. Plant Breeding. 4th Ed. 463 pp., 20 figs., 1907. 
Bateson, W. The Progress of Genetics since the Rediscovery 



492 Plant Physiology 

of Mendel's Papers. Progressus rei botanicae. 1 : 368- 

418, 1907. 
Bateson, W. Mendel's Principles of Heredity. 396 pp., 6 pis., 

33 figs., 1909. (Contains an extensive bibliography up to 

the date of publication.) 
Blaringhem, L. Mutations et traumatismes. Bull. Sci. de la 

France et de la Belgique (1907) : 248 pp., 8 pis. 
Castle, W. E. A Mendelian View of Sex Heredity. . Science, 

N. S. 29 : 395-400. 
Correxs, C. Die Bestimmung und Vererbung des Geschlects. 

81 pp., 1907. 
Darwin, Charles. Variation of Animals and Plants under 

Domestication. 1 : 473 pp. ; 2 : 478 pp. 
Davenport, E. The Principles of Breeding. 727 pp., 52 figs., 

1907. 
Davenport, C. B. Statistical Methods with Special Reference 

to Biological Variation. 2d Ed. 223 pp., 1904. 
East, E. M. The Relation of Certain Biological Principles to 

Plant Breeding. Conn. Agl. Exp. Sta. Bui. 158 : 91 pp., 

1907. 
Galton, F. The Average Contribution of Each Several An- 
cestor to the Total Heritage of the Offspring. Proc. Roy. 

Soc. 61:401-413, 1897. 
Johannsen, W. Uber Erblichkeit in Populationen und in 

reinen Linien. 1903. Jena. 
Lock, R. H. Recent Progress in the Study of Variation, 

Heredity, and Evolution. 299 pp., 1906. Xew York. 
Lotsy, J. P. Vorlesungen liber Deszendenztheorien mit be- 

sonderer Berucksichtigung der botanischen Seite der Frage. 

1906. Jena. 
MacDougal, D. T. Mutants and Hybrids of the (Enotheras. 

Carnegie Institution Publ. 24 : 57 pp., 13 figs., 1905. 
Plate, Ludwig. Selectionsprinzip und Probleme der Artbil- 

dung. 493 pp., 60 figs. Leipzig, 1908. 
Price, H. L., and Drinkard, A. W. Inheritance in Tomato 

Hybrids. Virginia Agl. Exp. Sta. Bui. 177 : 18-53, 10 pis., 

5 figs., 1908. 



Variation and Heredity 493 

Punnett, R. C. Mendelism. 85 pp., 1909. Cambridge. 

Reid, G. A. The Laws of Heredity. 548 pp., 25 figs., 1910. 

Shull, G. H. The Presence and Absence Hypothesis. Amer. 
Nat. 43 : 410-419, 1909. 

The Inheritance of Sex in Lychnis. Bot. Gaz. 49 : 110- 

125, 1910. 

Smith, L. H. Ten Generations of Corn Breeding. 111. Agl. Exp. 
Sta. Bui. 128 : 453-475, 1908. 

Vries, H. de. The Mutation Theory. 1:582 pp., 112 figs., 
! 4 col. pis., 1909 ; 2 : 683 pp., 149 figs., 6 col. pis., 1910. 

Species and Varieties. 847 pp.,- 1905. 

Plant Breeding. 360 pp., 114 figs., 1907. 

Wilson, E. B. Recent Researches on the Heredity and De- 
termination of Sex. Science, N. S. 29 : 53-71, 1909. 



CHAPTER XX 
GROWTH MOVEMENTS 

All plants possess the power of movement to at least a 
limited extent. The various types of movement and their 
relations constitute a considerable part of plant physiol- 
ogy as commonly presented. Here, however, it will be 
possible merely to outline portions of the subject, which 
may be further pursued in the special literature. It is 
entirely beyond the present purpose to consider locomotory 
movements, likewise the phenomenon of dehiscence, and 
other effects due to swelling and contraction. Neverthe- 
less, special movements of turgor are included on account 
of their closer relationship. 

Movement may occur within the protoplast, and may 
be limited to the cell, or it may occur in such manner that 
complex structures exhibit change of position or chanve 
in the direction of growth. As popularly regarded, those 
plants possessing roots are fixed in the soil or other sub- 
stratum, and movement is only associated with such 
striking changes as may be seen in the sensitive plant 
(Mimosa pudica), the Venus's-flytrap (LHonaea muscipula), 
or certain climbers and twiners. As a matter of fact, 
movement is almost inseparable from growth. 

The elongation of root or shoot is a type of growth 
movement. There is, furthermore, a remarkable variety 
494 



Growth Movements 495 

of growth responses resulting in curvature or orientation of 
members. As previously indicated, the movements of 
plant members are now regarded as primarily of two types, 
spontaneous (autonomic) and induced (paratonic). The 
former are little understood. It is not always possible 
to distinguish positively between the two types, or the 
movement may be the result of conjoint (internal and 
external) stimuli. In general, growth movement is a 
fundamental requirement in the effective adjustment of 
organisms to their environment. A study of the phenom- 
ena is more important educationally in liberalizing our 
views of plant relations than of any direct assistance in 
special problems of plant production. 

300. Stimulus and response. — The relations of or- 
ganisms to growth factors have been considered, but it is 
necessary to refer again to the environmental forces which 
condition plant activity. When the environment is 
favorable the plant is regarded as exhibiting the condition 
of tone ; and the effect of each factor or of the various 
factors severally is a tonic influence. The factors con- 
cerned are those essential in growth, especially oxygen, mois- 
ture, food-supply, light, and heat. They are sometimes 
known as the formal growth conditions. Some of these, 
and likewise other environmental factors, may act not as 
tonic influences but as special stimuli releasing growth 
responses, that is, movements. 

The plant is not merely a complex mechanism ; it is, 
when in a condition of tone, a source of readily releasable 
energy, — growth energy. It requires a stimulus to make 
this energy manifest, but it appears unnecessary that the 
stimulus should impart force. As so often pointed out, 



496 Plant Physiology 

the stimulus required is analogous to the pressure of the 
finger on the electric button which sets at work powerful 
dynamos. The pressure upon the button has no relation 
to the amount of work which will be accomplished by the 
machines thus released. When a stimulus affecting the 
plant is external, its relations to response may be worked 
out with a fair degree of success; but the methods of ac- 
tion of internal stimuli are almost entirely unknown. 
In the action of any stimulus upon a sensitive organ there 
are to be distinguished primarily 1 1 I perception, (2) trans- 
mission, and (3) reaction or growth response. Commonly 
the perceptive region is at no greal disl ance from the motor 
or responsive part ; yet in certain cases the stimulus may 
be transmitted through considerable intervening tissue. 
Practically nothing is known regarding tin- mechanism of 
transmission. Perception often reside- in the terminal 
portion of the organ, but not always in the formative 
region. 

In order to produce response a Btimulus must act 
usually for a certain interval of time, and this interval 
(presentation time) depend- upon temperature and other 
growth conditions. The visible response to the stimulus 
may be prompt, as in the case of many tendrils, or it may 
be delayed for several hours. The interval between stimu- 
lation and response (reaction time) is usually longer than 
that of presentation. 

301. Tropic curvatures. — Every plant exhibits a 
normal form and habit, and its members are arranged in a 
definite manner with respect to one another and to en- 
vironmental forces. Tropic curvatures are commonly 
the results of irritable growth responses manifest when the 



Growth Movements 497 

normal position of a sensitive structure is shifted, or when 
this member comes under the influence of new or intensified 
forces, or of forces acting from a new direction. The 
curvature of horizontally placed roots toward the earth 
and the bending of the hypocotyl of a seedling exposed to 
one-sided illumination are familiar illustrations. 

The importance of a means of orientation in order to 
assume or restore the normal is obvious. If the reaction 
of the growing organ results in orientation parallel to the 
direction of the exciting force, the organ is parallelotropic, 
while one assuming a position at an angle to the direction 
of the stimulus is plagiotropic, — diatropic being at right 
angles to the path of the stimulus. The tropic movements 
here discussed are effected in growing structures, or those 
in which growth may be initiated, and they cease with 
incapacity for growth. Moreover, the stimulus remaining 
the same, the rapidity of growth determines the prompt- 
ness of the response, or reaction time. 

Attention has been directed to the tropic responses of 
plants to light (phototropism) and to heat (thermotropism) . 
Other stimuli inducing reactions are such as gravity (ge- 
otropism), contact (thigmotropism), moisture (hydrotro- 
pism), electricity (electrotropism), and certain chemical 
agents (chemotropism) . 

302. Geotropism. — The vertical position of the main 
axis of most plants is as apparent as the erect posture of 
man. A seed may be planted in any position in the soil ; 
but as soon as sufficient growth is made the parallelotropic 
position of the axis is assumed, with the root directed 
straight downwards (positively geotropic) and the stem 
directly upwards (negatively geotropic). Germinating 
2k 



498 



Plant Physiology 



z 



seed in a moist chamber pinned in a horizontal posi- 
tion will respond to the stimulus of gravity by growth 
curvatures in the same manner. 
Secondary roots and branches take 
up plagiotropic positions, but in the 
remote branchesof the rool little geo- 
tropic response is manifest. Shoots 
from a fallen trunk assume tin- ver- 
tical position. If the terminal di<><»t 
of spruce i- cut of'f.oneor more Lateral 
shoots of the firsl whorl may be 
raised into the vertical position. 
The erection of the jointed stem- of 
grasses is effected by curvature- in 
the nodes, and these stems are par- 
ticularly interesting for Btudy. 
( reotropic response is not a ques- 

l HI \ .11 till - <»| .WW, ... . , 

VicUx Faba /horizontal tion of Weight, and this is shown by 
position 1 1 >. :it'i« i7 hra 
(II), and after 23 hra 
(III). [After Sachs and axis 

NoU-] there is no geotropic response when 

gravity is eliminated, as by revolving seedlings in a ver- 
tical plane on a klinostal geared to make one revolution 
in about fifteen minutes. On a klinostal rotated hori- 
zontally at a low rate of speed the usual stimulus of 
gravity is felt; hut when rotated at a higher rate of 
speed the root grows outward and toward the horizontal, 
and the shoot inward and toward the horizontal, de- 
pending upon the rati 1 of rotation. 

In the case of the root the perceptive region is usually 
confined to about one millimeter, i r less, at the very tip. 




curvature <>f root 



the diverse reaction- of the main 
and branches. Furthermore. 



Growth Movements 



499 



while curvature occurs in the 
region of greatest growth. 
The time required for per- 
ception (presentation time) 
varies from a few minutes to 
several hours. Reaction time 
is often several hours, and re- 
sponse is evident even if 
meanwhile the position of the 
organ is again shifted. The 
mechanism of geotropic per- 
ception is not clearly under- 
stood. An early mechanical 
theory of Knight has found 
new life in the statolith theory 
of Nemec, Haberlandt, and 
others. By this hypothesis it 
is assumed that in shifted 
structures the sinking of cer- 
tain cell products, especially 
starch, to the bottom of the 
cell produces a change of 
pressure, and it is this change 
which furnishes the excitation. 
303. Thigmotropism. — The 
capacity for thigmotropic re- 
sponse, or growth curvature 
induced by contact, is most 
highly developed in tendrils. 

Fig. 143. Diagrammatic new of geo- 
tropic curvature in an etiolated hypo- 
cotyl, horizontally placed. [After 
Noll.] 




500 



Plant Physiology 



Roots and leafy shoots appear to possess this power to a 

very limited extent. 

Plants producing ten- 
drils are particularly well 
equipped to climb aloft, 
supporting themselves by 
the attachment of these 
to any small supports, 
especially to those hori- 
zontally placed. By this 
means such plants as the 
grape vine, wild cucum- 
ber, Passiflora, and many 
ot hers are enabled to climb 
through trees and lay- 
ered vegetation, whereas 
twining plants commonly 
require a support which 
is more or less vertical. 
The tendrils are com- 
monly axillary or super- 
numerary branches devoid 
of leaves, or leaf-parts 
entirely lacking blade.-. 
Sometimes, however, pet- 
ioles of normal leaves or 
extended leaf tips mal- 
function as tendrils. 
These structures corn- 
exhibit dorsi- 
ventrality, and a right and 




Fig. 144. Demonstration klinostat, 
after Ganong, vertical arrangement. 
[Illustration from Bausch and Lomb rnonlv 
Optical Co.] 



Growth Movements 501 

left flank may be differentiated. They usually complete 
their growth within a few days, so that the plant may be 
attached to its supports almost as rapidly as the shoot 
elongates. 

The terminal part of the tendril is the more perceptive 
region, and commonly the under surface exhibits greater 
sensitiveness. Both surfaces and flanks may, however, 
respond to contact stimuli. When the tendril is from 
one-fourth to one-third grown, it exhibits marked auto- 
nomic nutations, and the swinging of the tip through space 
brings it into contact with any objects in the range of this 
motion or of swaying movements caused by wind. Scrap- 
ing the surface of the tendril against a suitable support 
(especially repeated scraping) is followed by coiling and 
close attachment around the object. The tendril is now 
fixed at both ends, the prompt grasping of the support 
being in part, apparently, due to turgor movements. 
After attachment growth proceeds more rapidly on the 
upper surface, and the tensions resulting throw the tendril 
into a close coil, once or more reversed. 

Fixation by means of tendrils affords not merely secure 
support, but the attachment at many points affords a 
general elasticity and freedom from severe shock well 
known through the principle of vehicle and car springs. 

304. Chemotropism. — The curvature and growth of 
roots, pollen-tubes, or fungous hyphse in response to the 
stimulus of chemical agents is chemotropism. At one 
time it seemed that chemotropic response, especially 
positive chemotropism, might commonly determine the 
direction of growth in roots, penetration of parasitic fungi, 
and other phenomena. Further study has developed the 



502 Plant Physiology 

probability that positive chemotropism is not a highly 
developed response. It may occur in roots and pollen- 
tubes, although the evidence is not entirely convincing ; 
while serious doubt has been thrown upon the existence 
of positive chemotropism in fungous hyphae. 

305. Nutation. — The tips of growing axes or other 
plant members are not as a rule extended in a straight 
line. Instead, they nod here and there or commonly trace 
an irregular spiral, the projection of which yields a series 
of more or less circular or elliptical figures. This type of 
movement is called nutation (circumnutation). It was 
extensively studied by Darwin, and the main effects to- 
gether with some of the important relations were clearly 
set forth at that time. 

The type of curve varies with the growth relations. In 
stems which are radially symmetrical nutation results 
from unequal growth in the vertical segments. The 
effects produced are accounted for by greater growth in 
each segment successively around the stem. When 
asymmetry occurs, and especially in flattened or dorsi- 
ventral organs there is more likelihood that the movement 
will tend toward narrow ellipses or even the back-and-forth 
linear type. The extent of the movement depends upon 
the unevenness and rapidity of growth. It is generally 
greatest in organs growing rapidly, such as tendrils and 
climbing shoots, and the whole of the growing region may 
be involved. Nevertheless, the pronounced nutation of 
twiners does not begin, as a rule, until after a fewinternodes 
are produced. Tendrils, likewise, show little nutation 
during the early stages of growth, and the movement 
ceases in matured organs. 



Growth Movements 503 

" All stages are shown between trifling and pronounced 
nutation, according to the plant, to the stage of develop- 
ment, and to the external conditions. The curves are not 
always regular and similar, even when there is a pronounced 
tendency to linear, elliptical, or circular nodding, as the 
case may be. Even when the last named is most pro- 
nounced it may temporarily alter into to-and-fro pendulum 
movements." 1 

The stimulus to nutation is in most cases primarily 
internal and spontaneous, but it may be conditioned, 
initiated, or in large part induced by other agencies, espe- 
cially by gravity and light. The time required for the 
completion of a single ellipse, or back and forward move- 
ment, may be one or two hours or as many days ; and when 
there is a tendency toward the latter type of nutation, 
the movement of the organ is least rapid near the point 
of reversal. 

306. Nastic curvatures. — In most of the types of 
growth response already considered the stimulus is uni- 
lateral and the curvature may occur in any plane. Fairly 
well distinguished from the preceding are those cases in 
which the structure of the organ is such that response is 
usually limited to orientation in a single plane, whether 
the stimulus is diffuse or unilateral. Bilateral or dorsi- 
ventral members, such as leaves, floral leaves, and flat- 
tened stems, are structures of the type above noted. The 
benclings resulting in such organs are known as nastic 
curvatures, and they may be distinguished by the same 
prefixes as in the other cases to denote the type of stimu- 
lus, thus photonasty, thermonasty. 

1 Pfeffer (Ewart), Physiology, 3 : p. 20. 



504 Plant Physiology 

Nastic curvatures, however, are not necessarily the 
result of external stimuli, hence they may be either auto- 
nomic or paratonic. 

In the development of leaves (section 181) there is 
usually a growth response whereby the under or dorsal 
surface grows faster, yielding an upward curvature (hypo- 
nasty). As a result of this each leaf in turn becomes a 
part of the bud. Later the growth on the upper or ventral 
surface is more rapid and there is outward bending (epi- 
nasty) during exfoliation. There may be a recurrence of 
epinastic and hyponastic curvature under the influence 
of various stimuli until maturity of the leaf. Growth 
upon the upper surface called forth by light is a paratonic 
nastic bending, or photepinasty. 

307. Nyctitropism. -— The old idea of floral clocks was 
founded on the observation that flowers of diverse species 
open and close with different light and temperature 
relations. There are some flowers which remain closed 
during the night, opening in the early morning with in- 
creased temperature or sunshine. Others arc less readily 
stimulated and remain closed until the conditions are 
further intensified. Again, some blossom when the heat 
of the day begins to decline, while the night-blooming 
Cereus and certain other flowers bloom at night. 

Movements of floral leaves have been shown to be typi- 
cally nastic growth movements and they disappear as 
soon as the power of growth is lost in these organs, unless 
accompanied by special basal articulations which may 
show turgor movements. 

Quite as characteristic are the sleep movements of leaves 
in a number of families, especially Leguminosse and Mi- 



Growth Movements 505 

mosse. All plants possessing jointed leaves do not exhibit 
the same behavior. Nyctitropic movements are com- 
monly due to changes of turgidity, and growth is not 
usually involved. The articulations are cushions in which 
cortical tissue predominates. Under stimulation the 
dorsal and ventral halves give osmotic changes unequal 
in rapidity so that movement is brought about. 

LABORATORY WORK 

Geotropism. — For a few observations upon the geotropism of 
roots fairly large seeds are desirable, such as those of peas or 
beans. Germinate the seed in moss or on paraffined wire netting 
over water. When germination has progressed to the extent of 
a few centimeters, the roots may be marked off with India ink 
as for determining the region of extension. The growth curva- 
tures are then to be followed by placing the radicles in a hori- 
zontal, or any other desired, position. If only a few seed are 
used, they may be pinned to the lower side of large corks covering 
jars or dishes partially filled with water. 

For a larger number of seed and particularly for observations 
respecting the effects on side roots, the seedlings may be arranged 
at various angles on two thicknesses of moistened carpet or felt 
paper between plates of glass. The plates are clamped together 
with wooden clothespins, and wads of filter-paper here and there 
prevent crushing. Place the plates on edge in a moist greenhouse 
or cover with wet cloths. Observe from time to time, note the 
results, and shift the position of the plates through ninety degrees 
after secondary roots are produced. Discuss the results. 

Negative geotropism of young shoots may be followed by ob- 
serving the behavior of bean or pea seedlings when the pots are 
placed horizontally. Determine also the time of presentation and 
of reaction for such seedlings grown in very small (2 inches) 
pots. Compare the presentation time at 12 to 15° with the 
interval at 25 to 30° C. 

Secure shoots of Tradescantia or of oats embracing several 



506 Plant Physiology 

nodes; pin the basal node to a cork or block of wood and follow 
the process of erection. 

With the special instructions given determine the behavior of 
roots and shoots of seedlings when gravity is equalized through 
vertical rotation upon the klinostat. 

Chemotropism. — The existence of positive and negative chem- 
otropism would seem to be established and some of the ehemo- 
tropic relations of pollen-tubes may be conveniently and easily 
observed. Utilize pollen known to germinate freely, such as 
that of Tradescantia virginica and Narcissus T<i:</t<i and prepare 
hanging-drop cultures a- for pollen germination. When the 
grains begin to germinate, introduce into the drops, bits of the 
stigma of the planl from which the pollen was taken. Ascer- 
tain if these stigma bits or if particles of any vegetable proteins 
(albumins and globulin- 1 exert any influence on the direction of 
growth of t la- tubes. 

If time for more extensive stud} is available, consult the paper 
by Lidforss (or follow special instructions . employ Pfeffer'i 
capillary tube method, and in-tall the accessary experiments. 

Growth "in/ movement of tendrils. -Utilizing any tendril- 
bearing planl available in the greenhouse <>r field, select Beveral 
tendrils about one fourth grown, mark off into ten or twenty 
spaces by means of India ink. and determine the region and 
period of growth, also the daily percentage increase in the differ- 
ent longitudinal segments. 

Review in suitable literature the more extensive accounts of 
tendril movements, and make an extended observation upon the 
behavior of one type, presenting the results in the form of a 
report. 

References 

Bose, J. C. Plant Response. 781 pp., 278 figs., 1906. 
Darwix, Charles. The Power of Movement in Plants. 592 

pp., 1885 [Appleton]. 
Darwix, Charles and Fr. The Movements and Habits of 

Climbing Plants. (2d Ed.) 208 pp., 1884 [Appleton]. 



Growth Movements 507 

Darwin, Fr. Lectures on the Physiology of Movement in Plants. 

New Phytologist. 5 and 6: (Lectures I-VI), 1906, 1907. 
Fitting, H. Weitere Unters. z. Physiologie der Ranken. 

Jahrb. f. wiss. Bot. 39 : 424-526, 21 figs., 1903. 

Die Reizleitungsvorgange bei den Pflanzen. 157 pp., 1907. 

Fulton, H. R. Chemotropism of Fungi. Bot. Gaz. 41 : 81- 

108, 1906. 
Haberlandt, G. Sinnesorgane im Pflanzenreich. 205 pp., 

1901. 
Zur Statolithentheorie des Geotropismus. Jahrb. f. wiss. 

Bot. 38 : 447-500, 1903. 
Kohl. Die Mechanic der Reizkrummungen. 1894. 
Lidforss, B. Unters. iiber Reizbewegung d. Pollenschlauche. 

Zeit Bot. 1 : 443-496, pi. 3, 1909. 
MacDougal, D. T. The Mechanism of Curvature of Tendrils. 

Ann. Bot. 10 : 373-402, 1 pi., 1896. 
Nemec, B. Die Reizleitung und die Reizleitende Strukturen. 

153 pp., 3 pis., 1901. 

Studien iiber die Regeneration. 387 pp., 180 ^s., 1905. 

Newcombe, F. C, and Rhodes, Anna L. Chemotropism of 

Roots. Bot. Gaz. 37 : 25-35, 1904. 
Peirce, G. J. A Contribution to the Physiology of the Genus 

Cuscuta. Ann. Bot. 8 : 53-118, 1 pi., 1894. 
Pfeffer, W. Die periodische Bewegungen der Blattorgane. 

176 pp., 1875. 

Irritability of Plants. Nature. 49 : 586-587, 1894. 

Pollock, J. B. The Mechanism of Root Curvature. Bot. 

Gaz. 19 : 1-80, 1900. 
Richter, O. Ueber das Zusammenwirken von Heliotropismus 

und Geotropismus. Jahrb. f. wiss. Bot. 4 : 481-502, 

1909. 
Schenk, A. Beitrage z. Biologie und Anatomie der Lianen. 

1892. 
Spalding, V. M. The Traumatropic Curvature of Roots. 

Ann. Bot. 8 : 423-450, 1894. 

Texts. Barnes, Detmer, Ganong, Jost, MacDougal, Pfeffer. 



INDEX 



Absorption, of carbon dioxid, 196 ; 

of oxygen, 285, 299 ; of water by 

leaves, 58, 62 ; principles of, 64. 
Acids, toxic action of, 114. 
Acton, E. H., 12. 
Adams, G. E., 168. 
Adventitious organs, 343. 
Alkalies, toxic action of, 443. 
Allelomorph, 481. 
Amides, 261. 

Ammonia compounds, 230. 
Ammonification, 231, 245. 
Amyloplasts, 21. 
Annuals, food storage, 252. 
Antitoxic action, 184. 
Arendt, 140. 
Armstrong, E. F., 279. 
Ash, composition of, 138 ; content, 

136, 140 ; effects of conditions 

upon, 139. 
Askenasy, E., 345. 
Atkinson, G. F., 248. 
Atwater, W. O., 248. 



Bailey, L. H., 12, 151, 302, 317, 

345, 434, 491. 
Balanced solutions, 184, 191. 
Balls, W. L., 414. 
Barlow, B., 248. 
Barnes, C. R., 12, 83, 115, 203, 225, 

249, 279, 346, 507. 
Bateson, W., 480, 491, 492. 
Bayliss, W. M., 279. 
Bell, J. M., 167. 
Benecke, W., 167. 
Benedict, H. M., 114. 



Bevan, E. J., 279. 

Beyerinck, M. N., 248. 

Biennials, food storage, 252. 

Blackman, F. F., 224, 303, 414. 

Blaringhem, L., 492. 

Bleeding, 77. 

Bolley, H. L., 461. 

Bose, J. C, 506. 

Boykin, E. B., 396. 

Breazeale, J. F., 167, 183. 

Bretschneider, 140. 

Brown, A. J., 76, 461 ; E., 49' 

H. T., 89, 224. 
Buchner, 303. 
Buds, resting, 314. 
Budding, 329. 
Burbank, L., 475. 
Burgerstein, A., 114. 
Bushee, G. L., 30. 
Busse, W. W., 345. 
Butschli, O., 33. 



Calcium, role of, 175. 

Cameron, F. H., 56, 167, 193. 

Candolle, A. P. de, 398. 

Carbohydrates, 254. 

Carbon content, 195. 

Carbon dioxid, amount in air, 210 ; 
from respiration, 285, 299 ; ex- 
cretion by roots, 161. 

Castle, W. E., 492. 

Cell, 15 ; division, 324 ; embry- 
onic, 17 ; forms, 24, 33 ; living, 
32; sap, 23; theory, 17; wall, 
18, 22. 

Cellulose, 258, 275. 

Central cylinder, 50. 



509 



510 



Index 



Cerny, 304. 

Chamberlain, C. J., 379. 

Chemotropism, 497, 501, 506. 

Chlorine, 181. 

Chlorophyll, decomposition of, 218 ; 
distribution, 200 ; properties, 
202 ; relation to light, 217 ; ex- 
traction of, 218. 

Chlorophyllous plants, 197. 

Chloroplasts, 21, 217. 

Chromoplasts, 21. 

Chromosomes, 487. 

Church, A. H., 379. 

Clapp, G. L., 114. 

Clark, J. F., 461. 

Claudel, 147. 

Clements, E. S., 114; F. E., 12, 
135, 433. 

Conduction, 272. 

Conservation, 1. 

Copeland. E. B., 114. 

Correns, C, 379, 492. 

Coulter, J. M., 12, 379. 

Cowles, H. C, 12. 

Crochetelle, 147. 

Crocker, W. f 461, 398. 

Crone's solution, 145. 

Crop, ecology, 7; growth, 118; 
improvement, 475 ; water re- 
quirements, 116; zones, 8. 

Cross, C. F., 279. 

Curtis, C. C, 12. 

Cuttings, 373. 

Cytoplasm, 19. 

Czapek, F., 224, 279. 

D 

Dandeno, J. B., 63. 

Darwin, Charles, 379, 492, 506; F., 

12, 115, 506, 507. 
Davenport, C. B., 414, 402, 490, 

492; E., 490, 492. 
Deherain, 279. 
Deleano, N. T., 183. 
Deleterious agents, 436. 



Denitrification, 235, 247. 

Detmer, W., 12, 13, 115, 168, 304, 

346, 380, 398, 507. 
Dewar flask, 291. 
Diffusion, 65 ; role of, 76. 
Digestion, 250, 205. 
Dixon, H. H., 115. 
Dominant, 481. 
Dox, A. W., 279. 
Drinkard, A. W., 492. 
Ducts, 28. 

Duggar, B. M., 193, 435, 462. 
Duvel, J. W. T., 398. 



East, E. M., 492. 

Eckerson, S. H., 83, 115. 

Ecology, 4 ; crop, 7. 

Effront. J., 279. 

Electrotropism, 497. 

Endodermis, 50. 

Environment, 5. 

Enzymes, 267; carbohydrate, 269; 

protein, 271. 
Ernst, A., 168. 
Escombe, I'.. 89, 224. 
Etherization, .;:;."). 34 L 
Evaporation, 99 ; excessive, 97. 
Evaporimeter, 99. 
Ewart, A. J.. 34, 115, 503. 
Ewert, R., 379. 



Factors, environmental, 5. 
Fats and oil.. 259, 276. 
Fermentation, 296 ; acetic, 299 ; 
alcoholic, 297, 302; Laetic, 297. 

Fertility, 152, 162. 

Fertilization, 354 ; cross and self, 

358. 
Fest, F., 168. 

Fibrovascular bundles, 106. 
Fippin, E. O., 63, 135, 151, 168. 
Fischer, A., 34; E., 279. 
Fitting, H., 507. 



Index 



511 



Flaccidity, 37. 

Fletcher, S. W., 379. 

Flower, morphology, 349, 377. 

Foods, temporary, 251. 

Forcing, 332 ; by warm water, 338, 

343. 
Formalin, toxic action of, 447. 
Formative region, 24, 50. 
Freeman, G. F., 85. 
Freidenfelt, T., 63. 
Fruit buds, 317. 
Fruit setting, 378. 
Fruiting and vegetation, 376. 
Fuhrmann, F., 303. 
Fulton, H. R., 507. 
Fungicides, 452. 



Gallagher, 56. 

Galton, F., 492. 

Ganong, W. F., 13, 34, 83, 115, 225, 
304, 346, 507. 

Gartner, 478. 

Gas exchange, 492. 

Genetics, 476. 

Geotropism, 492, 505. 

Germinators, 61. 

Goebel, K., 345. 

Goodale, G. L., 13, 63, 115. 

Grafting, 329. 

Gram-molecular solution, 68. 

Greeley, A. W., 71. 

Green, J. R., 13, 279. 

Growth, 305; cell, 322; embry- 
onic, 308 ; evidences of, 307 ; 
factors, 305; movements, 341, 
494 ; tissues, 342. 

Griiss, J., 345. 

Guttation, 97, 113. 

H 

Haberlandt, G., 13, 435, 507; F., 

345, 414. 
Half-shade, 424; crops, 425; fac- 



tors, 430 ; morphogenic effects, 

426 ; quality, 429. 
Hall, A. D., 168. 
Hansen, A., 13, 225. 
Hansteen, B., 193. 
Hard wheat production, 132. 
Harris, F. M., 92 ; J. A., 490. 
Harrison, F. C., 248. 
Harter, L. L., 115, 193. 
Hartig, R., 345. 
Heald, F. D., 462. 
Heat-release, 290, 300. 
Hedgecock, G. G., 63. 
Hedrick, U. P., 279. 
Heinrich, 56. 
Hellriegel, H., 248. 
Heredity, 463, 476, 490. 
Hertwig, O., 34. 
Hickman, J. F., 398. 
Hicks, G. H., 148. 
Hilgard, E. W., 63, 168. 
Hopkins, C. G., 153, 168, 399. 
Howard, W. L., 345. 
Hydrophyte, 131. 
Hydrostatic rigidity, 36. 
Hj^drotropism, 497. 
Hygrograph, 112. 

I 

Illuminating gas, toxic action of, 
450. 

Imbibition, 64, 79. 

Inheritance, tj'pes of, 479. 

Insecticides, 452. 

Inulin, 275. 

Iron, 180. 

Irrigation, 122; corn, 124; date- 
palm, 127; fruits, 123; wheat, 
125. 



Jenkins, 137. 
Jennings, H. S., 472. 
Jensen, C. H., 462. 
Johannsen, W., 303, 345, 492. 



512 



Index 



Johnson, S. W., 13, 168. 
Jones, L. R., 462. 
Jordan, D. S., 468. 
Jost, L., 13, 34, 63, 83, 135, 183 
225, 249, 279, 304, 346, 380, 507. 

K 

Kahlenberg, L., 83, 462. 

Karsten, 13. 

Kearney, T. H., 193. 

Kellerman, K., 447. 

Kellogg, 468. 

King, F. H., 101, 122, 135, 248 

Klebs, G., 379. 

Klocker, A., 303. 

Kniep, H., 435. 

Knight, 478; L. I., 462. 

Kny, 72. 

Kohl, 507. 

Kolreuter, 478. 

Koopman, K., 345. 

Kostytschew, S., 303. 

Kreusler, U., 225. 



Leaf, areas, measurement of, 110; 

extension, 342; structure, 113; 

venation, 108. 
Leaves, exfoliation of, 313; water 

absorption of, 58, 62. 
Leguminous tubercles, 236. 
Leucoplasts, 21. 
Lewis, C. I., 379. 
Lidforss, B., 379, 380, 507. 
Life zones, 8. 
Light, and blossoms, 434 ; artificial, 

420; energy, 212; injury, 418, 

434; intensity and quality, 215. 

418, 433 ; monochromatic. 422 ; 

perception, 416, 432; relations, 

415 ; requirements, 417. 
Lipman, J. G., 248, 249. 
Livingston, B. E., 83, 115. 
Lloyd, F. E., 115. 



Lock, R. H., 49. 

Lodeman, E. G., 462. 

Loeb, J., 34, 194. 

Loew, O., 183, 194, 462. 

Lotsy, J. P., 492. 

Ludwig, F., 490. 

Lutz, L., 248. 

Lyon, T. L., 63, 135, 151, 168, 399. 

M 

MacDonald, W., 135. 
MacDougal, D. T., 13, 135, 249 

279, 435, 492, 507. 
Magnesium, role of, 175 ; toxic re- 
lations, L85. 
Marchlewski, L., 225. 
Matthei, G. L. C, 224. 
May, D. \V.. 1«)4. 
McCool, M. M., 188. 
McLaughlin, W. W., 135. 
Mendel, 480. 

Mendelian characters, tomato, 486. 
Meristem, 17, 24. 
Mesophyte, 131. 
Merriam, C. H., 414. 
Metabolism, products of, 250. 
Meyer, A., 27!). 
Middle lamella. 22. 
Minder. F., 435. 

Mineral nutrients, 136; avail- 
ability of, 160; forms of, 148; 
injurious action, 1N4, 191; in 
rock, 150 ; in soils, 157 ; removed 
by crops, 153; roles of, 169; 
translocation of, 141. 
Mo bius, M., 380. 
Molisch, H., 435, 414. 
Moore, E., 346; G. T., 447. 
Mueller-Thurgau, 414. 
Mutation theory, 472. 
Mvcorhiza, 244. 

N 

Xastic curvatures, 503. 
Xathansohn, A., ^3. 



Index 



513 



Natural selection, 467. 

Nemec, B., 507. 

Newcombe, F. C, 507. 

Newman, 148. 

Nitrates, 229. 

Nitrification, 233, 246 ; conditions 

of, 234. 
Nitrifying organisms, 233. 
Nitrites, 229 ; soil, 233. 
Nitrogen, content, 227 ; electric 

fixation, 245 ; fixation, 236 ; 

fungi fixing, 242 ; organisms 

fixing, 236 ; relation, 226 ; soil, 

228 ; sources of, 244. 
Nobbe, 45, 147, 399. 
Noll, 13. 

Nuclear division, 324. 
Nucleus, 20. 
Nutation, 502. 
Nutrient solutions, 144 ; strength 

of, 146. 
Nyctitropism, 504. 

O 

Oglevee, C. S., 462. 

Oliver, G. W., 491. 

Organic acids, 261. 

Organic food, 195. 

Organic matter, rate of production, 

216. 
Origin of varieties, 469. 
Osborne, T. B., 279. 
Osmoscope, 80. 
Osmosis, 65 ; explanation of, 67 ; 

nutrient salts and, 73. 
Osmotic pressure, 81 ; role of, 76. 
Osterhout, W. J. V., 13, 71, 194. 
Overton, E., 83. 
Oxygen, in respiration, 285 ; and 

growth, 302. 



Paddock, W-, 279, 407. 
Paraffined basket, 163. 
2l 



Parenchyma, 25. 

Parthenocarpy, 369, 379. 

Parthenogenesis, 364. 

Pearson, K., 490. 

Peirce, G. J., 13, 248, 303, 507. 

Percival, 245. 

Perennials, food storage, 252. 

Periblem, 50. 

Pfeffer, W., 13, 34, 63, 83, 115, 168, 
183, 225, 249, 279, 346, 380, 435, 
503, 507. 

Pfeffer's solution, 146. 

Phosphorus, availability, 161 : role, 
171. 

Photosynthesis, 219 ; course of, 
204 ; demonstration of, 205 ; fac- 
tors, 204, 223, 224 ; rate of, 222. 

Phototropism, 497. 

Pieters, A. J., 399. 

Plasmolysis, 69, 80. 

Plastids, 21. 

Plate, L., 492. 

Plerome, 50. 

Poisons, 436. 

Polarity, 310. 

Pollination, 350 ; secondary effects 
of, 368. 

Pollock, J. B., 507. 

Population, 464. 

Potassium, pyrogallate, 221 ; role 
of, 172. 

Potted plants, water supply, 127. 

Prazmowski, A., 248. 

Precipitation, 118; annual, 119. 

Precipitation membrane, 80. 

Prescott, S. G, 299. 

Price, H. L., 492. 

Proteins, 259 ; classes of, 260. 

Protoplasm, 17 ; irritability of, 31 ; 
movement of, 29, 33 ; per- 
meability of, 74, 82, 276. 

Pruning and growth, 326. 

Punnett, R. G, 493. 

Pure lines, 471. 

Puriewitz, 303. 

Purity of gametes, 4.84. 



514 



Index 



Rane, F. W., 435. 

Raumer, v., 168, 183. 

Recessive, 481. 

Reed, H. S., 115, 183, 462. 

Reid, G. A., 493. 

Relation of Ca to Mg, 185, 192. 

Reproduction, 347 ; and heredity 
477; non-sexual, 372. 

Resins, occurrence, 264. 

Respiration, 200; aerobic, 284 
anaerobic, 294; and cell divi- 
sion, 326; demonstration of, 
282; of wounded plants, 290; 
result of, 281. 

Respiratory activity, 286. 

Rest period, 318. 

Rhodes, A. L., 507. 

Richards, H. M., 304, 462. 

Richter, ()., 507. 

Ringing, 114, 273. 

Root cap, 49, 62. 

Root exeretions, 447. 

Root hairs, 4."), 61. 

Root pressure, 77, 82. 

Root systems, 39, 60. 

Root tip, strueture of, 50, 62. 

Root tubercles, 247. 

Rooting habits, 41. 

Roots, acid exerction from, 161, 
167; corrosion by, 167; solvent 
action of, 160. 

Roots, elongation, 311, 341. 

Rossi, G. de. 249. 

Rotation, 30. 

Rotmistrov, V., 63. 

Ruhland. W., 83. 

Running out, 375. 

S 

Sachs, J., 13, 63, 115, 168. 
Saida, K., 249. 

Sand hills, vegetation of, 130. 
Sap pressure, 77, 82. 



Schenck, A., 13, 507. 

Schimper, A. F. W., 13, 135. 

Schreiner, O., 462. 

Schunck, C. A., 225. 

Scion propagation, 330. 

Sclerenchyma, 26. 

Secondary thickening, 320. 

Seed, buried, 391 ; delayed germi- 
nation, 392; habit, 347;. har- 
vesting, 3NS ; maturity. 385 ; 
production, 381 ; size and weight 
of, 393 ; vitality, 389. 

Seedlessness, 369. 

Segregation, 4s4. 

Selection, 188. 

Selective absorption, 75. 

Self-sterility. 362. 

Semi-permeable membrane, 65. 

Senn, G., 225. 

Shamel, A. D., 380, 399. 

Shantz, H. L., 135. 

Shrinkage. 82. 

Shull, G. H., 490, 493. 

Siemens, ('. W., 435. 

Sieve tubes, 28. 

Silicon, 182. 

Smith. J. W., 135; L. H., 493. 

Snow. L. M., 63. 

Sodium, 181. 

Soil, bacteria, 240; fertility, 152, 
102 ; particles, 52 ; texture and 
water capacity, 53. 

Solution cultures, 151, 165. 

Sorauer. P., 13, 63, 135. 

Spalding. V. M.. 63. 507. 

Species and varieties. 404. 

Starch. 207. 250. 274; digestion, 
277 ; test for. 222. 

Stem apex, 313 ; elongation of, 314, 
341. 

Sterome, 27. 

Stevens. W. C, 13, 63, 279, 346. 

Stewart, J. B.. 435. 

Stimulation. 451. 

Stimulus and response, 495. 

Stock and scion, 330. 



Index 



515 



Stoklasa, J., 168, 304. 

Stomata, 90 ; production of, 92. 

Stone, G. E., 435 ; W. L., 462. 

Storage products, 251. 

Strasburger, E., 13, 34, 346, 380. 

Streaming, protoplasmic, 29. 

Sugar, 207, 255, 275. 

Sulfur, 182. 

Swingle, W. T., 127, 414. 

Synthesis, 227. 



Tannins, 263. 

Temperature, adjustment to, 408 ; 
and buds, 411 ; and germination, 
412 ; and photosynthesis, 216 ; 
and production, 401 ; and root 
elongation, 413 ; control of, 407 ; 
inhibition by, 404 ; relations, 400, 
414 ; response, 409 ; units, 405. 

Temperatures, cardinal, 402 ; plant, 
408. 

Tendrils, 506. 

Ten Eyck, A. M., 63. 

Teodoresco, E., 435. 

Ternetz, C, 249. 

Thermotropism, 497. 

Thigmotropism, 497, 499. 

Timiriazeff, C, 225. 

Tissues, 16 ; differentiation of, 319. 

Tomato, Mendelian characters of, 
486. 

Tonic influence, 495. 

Tower, W. L., 474. 

Toxic agents, 436, 461 ; acids, 442 ; 
alkalies, 443 ; effects of solids, 
440, 461 ; effects of substratum, 
440; formalin, 447; illuminat- 
ing gas, 450 ; methods of action, 
441 ; organic compounds, 449 ; 
salts of heavy metals, 446. 

Trabut, L., 398. 

Tracheae, 28. 

Tracheids, 27. 

Translocation, 141, 250, 278. 



Transpiration, 84 ; amount of, 87, 
110; and evaporation, 99; and 
growth, 102 ; conditions affect- 
ing, 96, 111 ; indications of, 109; 
mechanism of, 88 ; modifications 
affecting, 94. 

Transeau, 100, 112. 

Transplanting and wilting, 340. 

Tropic curvatures, 496. 

True, R. H., 462. 

Tschermak, 480. 

Turgor, 71. 

Turpentine, occurrence, 264. 

U 

Unavailable water, 55, 62. 
Unproductiveness, 448. 



Van't Hoff, J. H., 83. 

Variation, 463, 489 ; fluctuating, 

466. 
Varieties and species, 464. 
Vegetation and fruiting, 376. 
Velamen, 58. 
Verworn, M., 14, 34. 
Vessels, 28. 
Vincent, C. C, 379. 
Vines, S. H., 14, 279. 
Volkens, G., 135. 
Voorhees, E. B., 168, 249. 
Vries, H. de, 83, 493. 

W 

Wachter, W., 83. 

Wager, H., 435. 

Waite, M. B., 380. 

Ward, H. M., 249. 

Warming, 14, 128, 135. 

Water, absorption, 39 ; content, 

35, 37, 59; conduction, 113; 

transport, 102, 109 ; variation 

in organs, 38. 
Water cultures, 142. 



516 



Index 



Water loss, control of, 93. 

Water requirements, 116. 

Waugh, F. A., 34(3. 

Webber, H. J., 396. 

Weeds, destruction by poison.-, 154. 

Wellington, R., 279. 

AVheats, hard, 9. 

Wheeler, H. J., 168. 

Whipple, O. B., 407. 

Whitney, M., 168, 435. 

Whitson, A. R., 248. 

Wickson, E. J., 135, 155. 

Widtsoe, J. A., 135. 

Wiegand, K. M., 414. 

Wiesner, J., 279. 

Wilcox, L. M., 135. 



Wilfarth, 168, 248. 
Wilson, E. B., 34, 493; 
Wilting, 37, 69, 80. 
Wimmer, 168. 
Winton, 137. 
Wollny, W., 135. 
Woods. A. F., 183. 
Woronin, M., 249. 



Xenia, 365, 379 ; false, 368. 
Xerophytes, 129. 



Zimmermann, A., 279. 



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